Refactor and doc package handling, module name mangling (#19821)

* Refactor and doc package handling, module name mangling

* Consolidate, de-duplicate and extend package handling
* Alter how duplicate module names of a package are handled
* Alter how module names are mangled
* Fix crash when another package is named 'stdlib' (test case added)
* Doc what defines a package in the manual

Modules with duplicate names within a package used to be given 'fake'
packages to resolve conflicts. That prevented the ability to discern if
a module belonged to the current project package or a foreign package.
They now have the proper package owner and the names are mangled in a
consistent manner to prevent codegen clashes.

All module names are now mangled the same. Stdlib was treated special
before, but now it is same as any other package. This fixes a crash
when a foreign package is named 'stdlib'.

Module mangling is altered for both file paths and symbols used by the
backends.

Removed an unused module name to package mapping that may have been
intended for IC. The mapping was removed because it wasn't being used
and was complicating the issue of package modules with duplicate names
not having the proper package owner assigned.

* Fix some tests

* Refactor `packagehandling`

* Remove `packagehandling.withPackageName` and its uses
* Move module path mangling from `packagehandling` to `modulepaths`
* Move `options.toRodFile` to `ic` to break import cycle

* Changed import style to match preferred style

Co-authored-by: quantimnot <quantimnot@users.noreply.github.com>

compiler/modulepaths.nim

@@ -10,100 +10,6 @@
 import ast, renderer, strutils, msgs, options, idents, os, lineinfos,
   pathutils
 
-when false:
-  const
-    considerParentDirs = not defined(noParentProjects)
-    considerNimbleDirs = not defined(noNimbleDirs)
-
-  proc findInNimbleDir(pkg, subdir, dir: string): string =
-    var best = ""
-    var bestv = ""
-    for k, p in os.walkDir(dir, relative=true):
-      if k == pcDir and p.len > pkg.len+1 and
-          p[pkg.len] == '-' and p.startsWith(pkg):
-        let (_, a, _) = getPathVersionChecksum(p)
-        if bestv.len == 0 or bestv < a:
-          bestv = a
-          best = dir / p
-
-    if best.len > 0:
-      var f: File
-      if open(f, best / changeFileExt(pkg, ".nimble-link")):
-        # the second line contains what we're interested in, see:
-        # https://github.com/nim-lang/nimble#nimble-link
-        var override = ""
-        discard readLine(f, override)
-        discard readLine(f, override)
-        close(f)
-        if not override.isAbsolute():
-          best = best / override
-        else:
-          best = override
-    let f = if subdir.len == 0: pkg else: subdir
-    let res = addFileExt(best / f, "nim")
-    if best.len > 0 and fileExists(res):
-      result = res
-
-when false:
-  proc resolveDollar(project, source, pkg, subdir: string; info: TLineInfo): string =
-    template attempt(a) =
-      let x = addFileExt(a, "nim")
-      if fileExists(x): return x
-
-    case pkg
-    of "stdlib":
-      if subdir.len == 0:
-        return options.libpath
-      else:
-        for candidate in stdlibDirs:
-          attempt(options.libpath / candidate / subdir)
-    of "root":
-      let root = project.splitFile.dir
-      if subdir.len == 0:
-        return root
-      else:
-        attempt(root / subdir)
-    else:
-      when considerParentDirs:
-        var p = parentDir(source.splitFile.dir)
-        # support 'import $karax':
-        let f = if subdir.len == 0: pkg else: subdir
-
-        while p.len > 0:
-          let dir = p / pkg
-          if dirExists(dir):
-            attempt(dir / f)
-            # 2nd attempt: try to use 'karax/karax'
-            attempt(dir / pkg / f)
-            # 3rd attempt: try to use 'karax/src/karax'
-            attempt(dir / "src" / f)
-            attempt(dir / "src" / pkg / f)
-          p = parentDir(p)
-
-      when considerNimbleDirs:
-        if not options.gNoNimblePath:
-          var nimbleDir = getEnv("NIMBLE_DIR")
-          if nimbleDir.len == 0: nimbleDir = getHomeDir() / ".nimble"
-          result = findInNimbleDir(pkg, subdir, nimbleDir / "pkgs")
-          if result.len > 0: return result
-          when not defined(windows):
-            result = findInNimbleDir(pkg, subdir, "/opt/nimble/pkgs")
-            if result.len > 0: return result
-
-  proc scriptableImport(pkg, sub: string; info: TLineInfo): string =
-    resolveDollar(gProjectFull, info.toFullPath(), pkg, sub, info)
-
-  proc lookupPackage(pkg, subdir: PNode): string =
-    let sub = if subdir != nil: renderTree(subdir, {renderNoComments}).replace(" ") else: ""
-    case pkg.kind
-    of nkStrLit, nkRStrLit, nkTripleStrLit:
-      result = scriptableImport(pkg.strVal, sub, pkg.info)
-    of nkIdent:
-      result = scriptableImport(pkg.ident.s, sub, pkg.info)
-    else:
-      localError(pkg.info, "package name must be an identifier or string literal")
-      result = ""
-
 proc getModuleName*(conf: ConfigRef; n: PNode): string =
   # This returns a short relative module name without the nim extension
   # e.g. like "system", "importer" or "somepath/module"
@@ -163,3 +69,18 @@ proc checkModuleName*(conf: ConfigRef; n: PNode; doLocalError=true): FileIndex =
     result = InvalidFileIdx
   else:
     result = fileInfoIdx(conf, fullPath)
+
+proc mangleModuleName*(conf: ConfigRef; path: AbsoluteFile): string =
+  ## Mangle a relative module path to avoid path and symbol collisions.
+  ##
+  ## Used by backends that need to generate intermediary files from Nim modules.
+  ## This is needed because the compiler uses a flat cache file hierarchy.
+  ##
+  ## Example:
+  ## `foo-#head/../bar` becomes `@foo-@hhead@s..@sbar`
+  "@m" & relativeTo(path, conf.projectPath).string.multiReplace(
+    {$os.DirSep: "@s", $os.AltSep: "@s", "#": "@h", "@": "@@", ":": "@c"})
+
+proc demangleModuleName*(path: string): string =
+  ## Demangle a relative module path.
+  result = path.multiReplace({"@@": "@", "@h": "#", "@s": "/", "@m": "", "@c": ":"})