lib.filesystem.pathType: Fix for filesystem root argument

Previously this function couldn't handle / being passed, it would throw
an error:

error: attribute '' missing

       at nixpkgs/lib/filesystem.nix:24:20:

           23|   */
           24|   pathType = path: (readDir (dirOf path)).${baseNameOf path};
             |                    ^
           25|

Consequently this also fixes the
lib.filesystem.{pathIsDirectory,pathIsRegularFile} functions.

lib/filesystem.nix

@@ -22,7 +22,12 @@ in
     Returns the type of a path: regular (for file), symlink, or directory.
   */
   pathType = path:
-    (readDir (dirOf path)).${baseNameOf path};
+    # The filesystem root is the only path where `dirOf / == /` and
+    # `baseNameOf /` is not valid. We can detect this and directly return
+    # "directory", since we know the filesystem root can't be anything else.
+    if dirOf path == path
+    then "directory"
+    else (readDir (dirOf path)).${baseNameOf path};
 
   /*
     Returns true if the path exists and is a directory, false otherwise.

lib/tests/filesystem.sh

@@ -46,6 +46,7 @@ checkPathType() {
     fi
 }
 
+checkPathType "/" '"directory"'
 checkPathType "$PWD/directory" '"directory"'
 checkPathType "$PWD/regular" '"regular"'
 checkPathType "$PWD/symlink" '"symlink"'
@@ -62,6 +63,7 @@ checkPathIsDirectory() {
     fi
 }
 
+checkPathIsDirectory "/" "true"
 checkPathIsDirectory "$PWD/directory" "true"
 checkPathIsDirectory "$PWD/regular" "false"
 checkPathIsDirectory "$PWD/symlink" "false"
@@ -79,6 +81,7 @@ checkPathIsRegularFile() {
     fi
 }
 
+checkPathIsRegularFile "/" "false"
 checkPathIsRegularFile "$PWD/directory" "false"
 checkPathIsRegularFile "$PWD/regular" "true"
 checkPathIsRegularFile "$PWD/symlink" "false"