girepository: Re-number GIR file from 2.0 to 3.0

The library shipped by gobject-introspection.git was
`libgirepository-1.0.so`, but for some reason (accident?), it was
accompanied by `GIRepository-2.0.gir`. That’s been the case for the last
6 years.

In moving libgirepository to glib.git, we’ve bumped the version to
`libgirepository-2.0.so`, and have changed the API.

In order to avoid a collision between the new `GIRepository-2.0.gir` and
the old `GIRepository-2.0.gir`, we can either:
 * Rename the basename of the library (confusing).
 * Re-version the whole thing to 3.0 (would mean it’s completely out of
   sync with the rest of glib.git, and would lead to build system
   misery).
 * Re-version only the GIR file (a bit confusing, but hopefully less
   confusing).

So I’ve done the final option: glib.git now ships
`libgirepository-2.0.so` and `GIRepository-3.0.gir`. This avoids
collisions with what’s shipped by gobject-introspection.git, while
hopefully still making some sense.

We considered using version number 2.1 rather than 3.0, but decided
against it because that makes it look like it’s compatible with version
2.0, which it isn’t.

Note that none of these changes touch the
`${prefix}/lib/girepository-1.0` and `${prefix}/share/gir-1.0`
directories. The version numbers in those refer to the versions of the
GIR and typelib file formats, which have not changed.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3155

girepository/meson.build

@@ -242,7 +242,7 @@ if enable_gir
   girepository_gir = gnome.generate_gir(libgirepository,
     sources: libgirepository_gir_sources,
     namespace: 'GIRepository',
-    nsversion: '2.0',
+    nsversion: '3.0',
     identifier_prefix: 'GI',
     symbol_prefix: 'gi',
     export_packages: libgirepository_gir_packages,