subrepo: fix normalizing paths with scheme

After revision 0afe96e374a7, subrepo paths were normalized using
posixpath.normpath and that resulted in ssh paths being wrongly converted
from ssh://host/path to ssh:/host/path

This fix applies the same logic used in urlutil.url to split the path scheme
from the rest and only use posixpath.normpath to the string after scheme://

mercurial/subrepoutil.py

@@ -421,7 +421,18 b' def repo_rel_or_abs_source(repo):'
 
     chunks.reverse()
     path = posixpath.join(*chunks)
-    return posixpath.normpath(path)
+    matchscheme = re.compile(b'^[a-zA-Z0-9+.\\-]+:').match
+    if matchscheme(path):
+        scheme, path = path.split(b':', 1)
+        if path.startswith(b'//'):
+            path = path[2:]
+            sep = b'//'
+        else:
+            sep = b''
+        normalized_path = scheme + b':' + sep + posixpath.normpath(path)
+    else:
+        normalized_path = posixpath.normpath(path)
+    return normalized_path
 
 
 def reporelpath(repo: "localrepo.localrepository") -> bytes: