lib,doc: remove obvious usages of toString on paths

It gives a warning on the lazy-trees branch of Nix
(NixOS/nix#6530)

one of these was also giving me an error (the one in lib/trivial probably)

```
$ nix build
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at /home/artturin/nixgits/my-nixpkgs/lib/modules.
nix:349:99
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/.git' and then accessing it is dep
recated, at /home/artturin/nixgits/my-nixpkgs/lib/sources.nix:35:32
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc.nix'
and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/system/etc/etc-activ
ation.nix' and then accessing it is deprecated, at «stdin»:0
warning: applying 'toString' to path '/home/artturin/nixgits/my-nixpkgs/nixos/modules/installer/sd-card/sd
-image-aarch64.nix' and then accessing it is deprecated, at «stdin»:0
error: cannot decode virtual path '/nix/store/virtual0000000000000000000000005-source'
(use '--show-trace' to show detailed location information)
```

doc/doc-support/lib-function-locations.nix

@@ -38,7 +38,7 @@ let
       substr = builtins.substring prefixLen filenameLen filename;
       in substr;
 
-  removeNixpkgs = removeFilenamePrefix (builtins.toString pkgs.path);
+  removeNixpkgs = removeFilenamePrefix pkgs.path;
 
   liblocations =
     builtins.filter

lib/sources.nix

@@ -140,7 +140,7 @@ let
       origSrc = if isFiltered then src.origSrc else src;
     in lib.cleanSourceWith {
       filter = (path: type:
-        let relPath = lib.removePrefix (toString origSrc + "/") (toString path);
+        let relPath = lib.removePrefix (origSrc + "/") (path);
         in lib.any (re: match re relPath != null) regexes);
       inherit src;
     };
@@ -175,12 +175,12 @@ let
   */
   commitIdFromGitRepo =
     let readCommitFromFile = file: path:
-        let fileName       = toString path + "/" + file;
-            packedRefsName = toString path + "/packed-refs";
+        let fileName       = path + "/" + file;
+            packedRefsName = path + "/packed-refs";
             absolutePath   = base: path:
               if lib.hasPrefix "/" path
               then path
-              else toString (/. + "${base}/${path}");
+              else /. + "${base}/${path}";
         in if pathIsRegularFile path
            # Resolve git worktrees. See gitrepository-layout(5)
            then
@@ -226,7 +226,7 @@ let
 
   pathHasContext = builtins.hasContext or (lib.hasPrefix storeDir);
 
-  canCleanSource = src: src ? _isLibCleanSourceWith || !(pathHasContext (toString src));
+  canCleanSource = src: src ? _isLibCleanSourceWith || !(pathHasContext src);
 
   # -------------------------------------------------------------------------- #
   # Internal functions

lib/trivial.nix

@@ -213,8 +213,8 @@ rec {
     # Default value to return if revision can not be determined
     default:
     let
-      revisionFile = "${toString ./..}/.git-revision";
-      gitRepo      = "${toString ./..}/.git";
+      revisionFile = ./.. + "/.git-revision";
+      gitRepo      = ./.. + "/.git";
     in if lib.pathIsGitRepo gitRepo
        then lib.commitIdFromGitRepo gitRepo
        else if lib.pathExists revisionFile then lib.fileContents revisionFile