Consolidate warning location formatting

intrp: Consolidate subproject dep checking and logging

This already reports the location (in a slightly different format), but
using mlog.warning() will make it easier if we want to change the location
format in future.

These are the remaining warnings in the parser, where we have the location
to hand.

Add filename and lineno to duplicate kwargs warning

If a dep is not found on the system and a fallback is specified, we
have two cases:

1. Look for the dependency in a pre-initialized subproject
2. Initialize the subproject and look for the dependency

Both these require version comparing, ensuring the fetched variable
is a dependency, and printing a success message, erroring out, etc.

Now we share the relevant code instead of duplicating it. It already
diverged, so this is a good thing.

As a side-effect, we now log fallback dependencies in the same format
as system dependencies:

    Dependency libva found: YES
    Dependency libva found: YES (cached)

    Dependency glib-2.0 from subproject subprojects/glib found: YES
    Dependency glib-2.0 from subproject subprojects/glib found: YES (cached)

pkgconfig: Allow passing Dependency objects to library(_private)

Currently, this test only tries to load a predefined icon, so the resource
compilation and linking can be completely removed from the meson.build and
it still passes.

Change to try to load the icon contained in the resources we compile.