github: Map 'subscribers' instead of 'watchers' to ForgeFed 'followers'

On Github's API, 'watchers' is synonymous with 'stargazers', which does NOT map
to 'watching' on Github's UI.

swh/indexer/metadata_dictionary/github.py

@@ -153,12 +153,12 @@ class GitHubMapping(BaseExtrinsicMapping, JsonMapping):
             )
 
     @produce_terms(ACTIVITYSTREAMS.followers, ACTIVITYSTREAMS.totalItems)
-    def translate_watchers_count(self, graph: Graph, root: BNode, v: Any) -> None:
+    def translate_subscribers_count(self, graph: Graph, root: BNode, v: Any) -> None:
         """
 
         >>> graph = Graph()
         >>> root = URIRef("http://example.org/test-software")
-        >>> GitHubMapping().translate_watchers_count(graph, root, 42)
+        >>> GitHubMapping().translate_subscribers_count(graph, root, 42)
         >>> prettyprint_graph(graph, root)
         {
             "@id": ...,

swh/indexer/tests/metadata_dictionary/test_github.py

@@ -101,7 +101,7 @@ def test_compute_metadata_github():
   "visibility": "public",
   "forks": 1,
   "open_issues": 0,
-  "watchers": 13,
+  "watchers": 12,
   "default_branch": "master",
   "temp_clone_token": null,
   "organization": {
@@ -140,7 +140,7 @@ def test_compute_metadata_github():
         "as:followers": {
             "as:totalItems": {
                 "type": "xsd:nonNegativeInteger",
-                "@value": "12",
+                "@value": "6",
             },
             "type": "as:Collection",
         },