This fix a regression caused by
https://github.com/mesonbuild/meson/pull/7837, it wanted to make
InternalDependency's external_deps private but has side effect of making
StatisLibrary's external_deps private too.

It is technically correct to make them private, but Meson used to make
StaticLibrary deps public so they are usable without `pkg-config
--static` when we know there is only a static library available.

Fixes: #7929.

Fixes #7925

It's an easy mistake to do this:

    gir1 = gnome.generate_gir(...)
    gir2 = gnome.generate_gir(...
        includes: ['GObject-2.0', gir1])

This fails with an error:

    ERROR: Gir includes must be str, GirTarget, or list of them.

The issue is that the 'gir1[0]' should be passed instead of 'gir1'.
To make the problem slightly clearly, tweak the error message to be:

    ERROR: Gir includes must be str, GirTarget, or list of them. Got TypelibTarget.

This fixes glib-2.0-uninstalled.pc file.
GLib does `extra_cflags : ['-I${libdir}/glib-2.0/include']` because some
of its headers gets installed there. But when used uninstalled that path
makes no sense and pkg-config aborts because ${libdir} is not defined.
This cannot be worked around by GLib because Meson does not allow
setting different `extra_cflags` for -uninstalled.pc, and does not allow
setting libdir in `uninstalled_variables`.

When glib is a subproject we should use glib-compile-resources it
overrides using find_program() in the case it is not installed on the
build machine. With old glib version we have to run
glib-compile-resources at configure time to generate the list of
dependencies, but not when glib is recent enough.

This fixes a regression introduced by
https://github.com/mesonbuild/meson/pull/7488.

InternalDependency's ext_deps previously where simply ignored, but that
PR has effect to add many more public Requires in generated pc files.

Share common code to extract the `variables` kwarg in
declare_dependency() and pkg.generate().

Closes #7839