Windows: Symlink support

To provide Windows-compatible functionality, spack code should use
llnl.util.symlink instead of os.symlink. On non-Windows platforms
and on Windows where supported, os.symlink will still be used.

Use junctions when symlinks aren't supported on Windows (#22583)

Support islink for junctions (#24182)

Windows: Update llnl/util/filesystem

* Use '/' as path separator on Windows.
* Recognizing that Windows paths start with '<Letter>:/' instead of '/'

Co-authored-by: lou.lawrence@kitware.com <lou.lawrence@kitware.com>
Co-authored-by: John Parent <john.parent@kitware.com>

lib/spack/external/macholib/util.py

@@ -6,6 +6,8 @@
 
 from macholib import mach_o
 
+from llnl.util.symlink import symlink
+
 MAGIC = [
     struct.pack("!L", getattr(mach_o, "MH_" + _))
     for _ in ["MAGIC", "CIGAM", "MAGIC_64", "CIGAM_64"]
@@ -140,7 +142,7 @@ def mergetree(src, dst, condition=None, copyfn=mergecopy, srcbase=None):
         try:
             if os.path.islink(srcname):
                 realsrc = os.readlink(srcname)
-                os.symlink(realsrc, dstname)
+                symlink(realsrc, dstname)
             elif os.path.isdir(srcname):
                 mergetree(
                     srcname,