primop: add readFileType, optimize readDir

Allows checking directory entry type of a single file/directory.

This was added to optimize the use of `builtins.readDir` on some
filesystems and operating systems which cannot detect this information
using POSIX's `readdir`.

Previously `builtins.readDir` would eagerly use system calls to lookup
these filetypes using other interfaces; this change makes these
operations lazy in the attribute values for each file with application
of `builtins.readFileType`.

(cherry picked from commit 153ee460c5)

tests/lang/eval-okay-readDir.exp

@@ -1 +1 @@
-{ bar = "regular"; foo = "directory"; }
+{ bar = "regular"; foo = "directory"; ldir = "symlink"; linked = "symlink"; }

tests/lang/eval-okay-readFileType.exp

@@ -0,0 +1 @@
+{ bar = "regular"; foo = "directory"; ldir = "symlink"; linked = "symlink"; }

tests/lang/eval-okay-readFileType.nix

@@ -0,0 +1,6 @@
+{
+  bar    = builtins.readFileType ./readDir/bar;
+  foo    = builtins.readFileType ./readDir/foo;
+  linked = builtins.readFileType ./readDir/linked;
+  ldir   = builtins.readFileType ./readDir/ldir;
+}

tests/lang/readDir/ldir

@@ -0,0 +1 @@
+foo

tests/lang/readDir/linked

@@ -0,0 +1 @@
+foo/git-hates-directories