Fix path resolution of submodules in the std namespace (#8453)

compiler/options.nim

@@ -556,13 +556,28 @@ proc findFile*(conf: ConfigRef; f: string; suppressStdlib = false): string {.pro
           result = rawFindFile2(conf, f.toLowerAscii)
   patchModule(conf)
 
+const stdlibDirs = [
+  "pure", "core", "arch",
+  "pure/collections",
+  "pure/concurrency", "impure",
+  "wrappers", "wrappers/linenoise",
+  "windows", "posix", "js"]
+
 proc findModule*(conf: ConfigRef; modulename, currentModule: string): string =
   # returns path to module
   const pkgPrefix = "pkg/"
-  let m = addFileExt(modulename, NimExt)
+  const stdPrefix = "std/"
+  var m = addFileExt(modulename, NimExt)
   if m.startsWith(pkgPrefix):
     result = findFile(conf, m.substr(pkgPrefix.len), suppressStdlib = true)
   else:
+    if m.startsWith(stdPrefix):
+      let stripped = m.substr(stdPrefix.len)
+      for candidate in stdlibDirs:
+        let path = (conf.libpath / candidate / stripped)
+        if fileExists(path):
+          m = path
+          break
     let currentPath = currentModule.splitFile.dir
     result = currentPath / m
     if not existsFile(result):