Improve getNimPrefixDir to support choosenim and env var.

See comment for more information. This commit also adds support
for an environment variable so that users have a workaround
when Nimble cannot find the Nim stdlib.

readme.markdown

@@ -773,6 +773,13 @@ installation.
 
 Make sure that you are running at least version 0.16.0 of Nim (or the latest nightly).
 
+* ``Error: cannot open '/home/user/.nimble/lib/system.nim'.``
+
+Nimble cannot find the Nim standard library. This is considered a bug so
+please report it. As a workaround you can set the ``NIM_LIB_PREFIX`` environment
+variable to the directory where ``lib/system.nim`` (and other standard library
+files) are found.
+
 ## Repository information
 
 This repository has two main branches: ``master`` and ``stable``.

src/nimblepkg/nimscriptsupport.nim

@@ -188,7 +188,20 @@ proc setupVM(module: PSym; scriptName: string, flags: Flags): PEvalContext =
       else:
         flags[key] = @[value]
 
-proc getNimPrefixDir(): string = splitPath(findExe("nim")).head.parentDir
+proc getNimPrefixDir(): string =
+  let env = getEnv("NIM_LIB_PREFIX")
+  if env != "":
+    return env
+
+  result = splitPath(findExe("nim")).head.parentDir
+  # The above heuristic doesn't work for 'choosenim' proxies. Thankfully in
+  # that case the `nimble` binary is beside the `nim` binary so things should
+  # just work.
+  if not dirExists(result / "lib"):
+    # By specifying an empty string we instruct the Nim compiler to use
+    # getAppDir().head as the prefix dir. See compiler/options module for
+    # the code responsible for this.
+    result = ""
 
 when declared(ModuleGraph):
   var graph: ModuleGraph