Some fallbacks are broken

While adding CI for MSVC and android/mingw cross compilation, I've noticed pygobject fails to build as subproject.

It can be tested on Linux by configuring gst-build with --wrap-mode=forcefallback, we should probably have a CI flavour that does that.

|WARNING: Dependency gobject-introspection not found but it is available in a sub-subproject.
|To use it in the current project, promote it by going in the project source
|root and issuing the following command: 
|meson wrap promote subprojects/pygobject/subprojects/gobject-introspection.wrap
|Couldn't use fallback subproject in subprojects/gobject-introspection for the dependency gobject-introspection-1.0 
|Reason: Failed to initialize 'subprojects/gobject-introspection':
|Subproject directory not found and gobject-introspection.wrap file not found
|fallback for gobject-introspection-1.0 not found

In gst-python:

|Could not find dependency pygobject_dep in subproject pygobject

In gst-libav:

|Couldn't use fallback subproject in subprojects/FFmpeg for the dependency libavfilter 
|Reason: Failed to initialize 'subprojects/FFmpeg':
|Subproject directory not found and FFmpeg.wrap file not found
|fallback for libavfilter not found