Added a cache in library_url

None is a valid value, so to distinguish between it and "_library_url hasn't
been computed yet", I check the attribute's presence.

shpc/main/registry/remote.py

@@ -126,16 +126,21 @@ def library_url(self):
         """
         Retrieve the web url, either pages or (eventually) custom.
         """
+        if hasattr(self, "_library_url"):   # Cannot do "if self._library_url" because None is a valid value
+            return self._library_url
         url = self.url
         if url.endswith(".git"):
             url = url[:-4]
         parts = url.split("/")
         domain = parts[2]
         if domain in self.library_url_schemes:
-            return self.library_url_schemes[domain] % (
+            self._library_url = self.library_url_schemes[domain] % (
                 parts[3],
                 "/".join(parts[4:]),
             )
+        else:
+            self._library_url = None
+        return self._library_url
 
     def exists(self, name):
         """
@@ -184,11 +189,10 @@ def _update_cache(self, force=False):
         if self._cache and not force:
             return
 
-        library_url = self.library_url
-        if library_url is None:
+        if self.library_url is None:
             return self._update_clone_cache()
         # Check for exposed library API on GitHub or GitLab pages
-        response = requests.get(library_url)
+        response = requests.get(self.library_url)
         if response.status_code != 200:
             return self._update_clone_cache()
         self._cache = response.json()