meson: List libraries and their corresponding gir definition

meson: List libraries and their corresponding gir definition

Introduces a `libraries` variable that contains all libraries in a
list with the following format:

``` meson
libraries = [
    [pkg_name, {
        'lib': library_object
        'gir': [ {full gir definition in a dict } ]
    ],
    ....
]
```

It therefore refactors the way we build the gir so that we can reuse the
same information to build them against 'gstreamer-full' in gst-build
when linking statically

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1093>