doc: Adapt GLib documentation path to GLib 2.80.1

With GLib 2.80.1 (commit 548ec9f1), the installation paths for the documentation
have changed once again:

    The `gi-docgen` tool is not designed to be used like that. In
    particular, when nesting documentation directories, the generated
    `*.devhelp2` files (needed by Devhelp to show the documentation) are
    nested one directory level too deep for Devhelp to find them, and hence
    are useless, and the documentation doesn’t show up in this common
    documentation viewer.

    So, change the installed documentation directory hierarchy:
     * `${PREFIX}/share/doc/glib-2.0/gio` → `${PREFIX}/share/doc/gio-2.0`
     * `${PREFIX}/share/doc/glib-2.0/glib-unix` →
       `${PREFIX}/share/doc/glib-unix-2.0`
     * `${PREFIX}/share/doc/glib-2.0/gobject` →
       `${PREFIX}/share/doc/gobject-2.0`
     * etc.
     * `${PREFIX}/share/doc/glib-2.0/glib` → `${PREFIX}/share/doc/glib-2.0`

https://gitlab.gnome.org/GNOME/glib/-/issues/3287
Reimplements #618

meson.build

@@ -73,12 +73,19 @@ if with_docs
 
   if glib.version().version_compare('<2.79.0')
     glib_docpath = join_paths(glib_prefix, 'share', 'gtk-doc', 'html')
+    glib_modules = ['glib', 'gobject' ]
   else
-    glib_docpath = join_paths(glib_prefix, 'share', 'doc', 'glib-2.0')
     warning('glib >= 2.79.0 documention might not be properly referred from libmodulemd documentation.')
+    if glib.version().version_compare('<2.80.1')
+      glib_docpath = join_paths(glib_prefix, 'share', 'doc', 'glib-2.0')
+      glib_modules = ['glib', 'gobject' ]
+    else
+      glib_docpath = join_paths(glib_prefix, 'share', 'doc')
+      glib_modules = ['glib-2.0', 'gobject-2.0' ]
+    endif
   endif
 
-  foreach referred_module : [ 'glib', 'gobject' ]
+  foreach referred_module : glib_modules
     doc_module_path = join_paths(glib_docpath, referred_module)
     doc_index_file = join_paths(doc_module_path, 'index.html')
     ret = run_command ([test, '-e', doc_index_file],