girepository: Make gi_repository_find_by_gtype() deterministic

When faced with a `GType` which is present in multiple typelibs, the old
implementation was not deterministic, as it iterated over a hash table
of typelibs. The iteration order of a hash table is not deterministic.

Use the new `ordered_typelibs` and `ordered_lazy_typelibs` arrays to
iterate instead, making the order deterministic.

Add a unit test to check this. In particular, to check that symbols
which are present in both `Gio` and `GioUnix` are correctly resolved as
being from `GioUnix`.

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

Helps: #3303

girepository/introspection/meson.build

@@ -261,6 +261,7 @@ if host_system == 'windows'
       '--identifier-prefix=GWin32'
     ],
   )
+  gio_platform_gir = gio_win32_gir
 else
   gio_unix_gir_c_includes = []
   foreach h: gio_unix_include_headers
@@ -289,6 +290,7 @@ else
       '--identifier-prefix=GUnix'
     ],
   )
+  gio_platform_gir = gio_unix_gir
 endif
 
 # GIRepository