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>

tests/ccgbugs/tforward_decl_only.nim

@@ -1,7 +1,7 @@
 discard """
 ccodecheck: "\\i !@('struct tyObject_MyRefObject'[0-z]+' {')"
 ccodecheck: "\\i !@('mymoduleInit')"
-ccodecheck: "\\i @('mymoduleDatInit')"
+ccodecheck: "\\i @('atmmymoduledotnim_DatInit000')"
 output: "hello"
 """
 

tests/modules/a/module_name_clashes.nim

@@ -0,0 +1,8 @@
+# See `tmodule_name_clashes`
+
+import ../b/module_name_clashes
+type A* = object
+  b*: B
+
+proc print*(a: A) =
+  echo repr a

tests/modules/b/module_name_clashes.nim

@@ -0,0 +1,3 @@
+# See `tmodule_name_clashes`
+
+type B* = object

tests/modules/tmodule_name_clashes.nim

@@ -0,0 +1,16 @@
+discard """
+targets: "c"
+ccodecheck: "\\i @('atmaatsmodule_name_clashesdotnim_DatInit000')"
+ccodecheck: "\\i @('atmbatsmodule_name_clashesdotnim_DatInit000')"
+joinable: false
+"""
+
+# Test module name clashes within same package.
+# This was created to test that module symbol mangling functioned correctly
+# for the C backend when there are one or more modules with the same name in
+# a package, and more than one of them require module initialization procs.
+# I'm not sure of the simplest method to cause the init procs to be generated.
+
+import a/module_name_clashes
+
+print A()

tests/package/stdlib/system.nim

@@ -0,0 +1,2 @@
+# this module is part of tstdlib_name_not_special
+doAssert true

tests/package/tstdlib_name_not_special.nim

@@ -0,0 +1,3 @@
+# Test whether a another package named 'stdlib' can be imported and used.
+# This caused a crash in the past.
+import stdlib/system