Allow repeated invocations of 'mirror create'

When creating a cosmetic symlink for a resource in a mirror, remove
it if it already exists. The symlink is removed in case the logic to
create the symlink has changed.

lib/spack/spack/caches.py

@@ -70,7 +70,13 @@ def symlink(self, mirror_ref):
         relative_dst = os.path.relpath(
             mirror_ref.storage_path,
             start=os.path.dirname(cosmetic_path))
+
         if not os.path.exists(cosmetic_path):
+            if os.path.lexists(cosmetic_path):
+                # In this case the link itself exists but it is broken: remove
+                # it and recreate it (in order to fix any symlinks broken prior
+                # to https://github.com/spack/spack/pull/13908)
+                os.unlink(cosmetic_path)
             mkdirp(os.path.dirname(cosmetic_path))
             os.symlink(relative_dst, cosmetic_path)