meson: List libraries and their corresponding gir definition
Introduces a `libraries` variable that contains all libraries in a
dict with the following format:
``` meson
'lib': library_object
'gir': {full gir definition in a dict }
```
It therefore refactors the way we build the git so that we can reuse the
same information to build them against 'gstreamer-full' in gst-build
when linking statically
Edited by Tim-Philipp Müller