Adapt process_changes.py to process gitlab based entries

Closes #944

user_manual/changes/README.md

@@ -13,3 +13,8 @@ The contents are used for two purposes:
 To see what should be documented in each YAML entry, please see
 [entry_schema.yaml](./entry_schema.yaml), which uses the [JSON
 Schema](https://json-schema.org/) schema description, but with YAML objects.
+
+* Files of the form `S123-456.yaml` are old style ticket entries.
+
+* Files in the `libadalang` and `langkit` directories, and of the form
+  `123.yaml` are new style Gitlab ticket entries.

user_manual/changes/process_changes.py

@@ -137,19 +137,31 @@ def get_entries(*args):
         entries_names = args
     else:
         entries_names = [
+            # Old style TN entries live in the base dir
             f for f in glob(P.join(ENTRIES_PATH, '*.yaml'))
             if P.basename(f) != 'entry_schema.yaml'
-        ]
+        ] + (
+            # Gitlab entries live in subdirs
+            glob(P.join(ENTRIES_PATH, "libadalang", "*.yaml"))
+            + glob(P.join(ENTRIES_PATH, "langkit", "*.yaml"))
+        )
 
     for fname in entries_names:
-        tn = P.basename(fname).split('.')[0]
+        # With this transformation:
+        # * Old style ticket numbers will look like S123-456
+        # * New style Gitlab ticket numbers will look like libadalang-123
+        tn = P.relpath(
+            fname, start=ENTRIES_PATH
+        ).split('.')[0].replace("/", "-")
+
         with open(fname, encoding="utf-8") as f:
             entry = yaml.safe_load(f)
             entry['tn'] = tn
             validate_entry(tn, entry)
             yield entry
 
 
+
 types_to_header = OrderedDict((
     ('api-change', 'breaking changes'),
     ('new-feature', 'new features'),