pkgconfig: Fix linking to a custom target

[xclaesse]

When generating pkgconfig file for a library that links to an
uninstalled static library built by custom_target() Meson was crashing
when trying to access some attributes that does not exist on that class.

Also fix is_internal() implementation, it only really make sense on a
CustomTargetIndex or if CustomTarget has only a single output.

[xclaesse]

This fix meson crash seen in GStreamer: https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/133#note_696355.

CC @nirbheek

[gdesmott]

Does not seem to work here: https://gitlab.freedesktop.org/gstreamer/gst-plugins-rs/-/issues/133#note_706950

[jpakkane]

The test failure is hard to pick up from the test output but fairly straightforward when run locally:

Failed test during install: 'test cases/common/45 pkgconfig-gen'
Reason: 
Extra file usr/lib/pkgconfig/simple6.pc found.

[codecov]

Codecov Report

Merging #7996 (eb8d9f0) into master (ee0baa9) will increase coverage by 0.02%.
The diff coverage is 71.42%.

@@            Coverage Diff             @@
##           master    #7996      +/-   ##
==========================================
+ Coverage   67.12%   67.15%   +0.02%     
==========================================
  Files         404      404              
  Lines       85763    85761       -2     
  Branches    18917    18915       -2     
==========================================
+ Hits        57568    57591      +23     
+ Misses      23665    23642      -23     
+ Partials     4530     4528       -2     

Impacted Files	Coverage Δ
mesonbuild/build.py	77.49% <60.00%> (+0.29%)	⬆️
mesonbuild/modules/pkgconfig.py	82.29% <100.00%> (+0.04%)	⬆️
mesonbuild/scripts/vcstagger.py	87.50% <0.00%> (-4.17%)	⬇️
modules/pkgconfig.py	82.29% <0.00%> (+0.04%)	⬆️
build.py	73.94% <0.00%> (+0.29%)	⬆️
modules/python.py	67.08% <0.00%> (+0.49%)	⬆️
mesonbuild/dependencies/base.py	85.67% <0.00%> (+0.59%)	⬆️
mesonbuild/minstall.py	67.87% <0.00%> (+0.72%)	⬆️
mesonbuild/dependencies/dev.py	68.35% <0.00%> (+1.19%)	⬆️
... and 1 more

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[xclaesse]

Rebased and hopefully fixed the CI. Targetting 0.60.3 because it's a bugfix.

[xclaesse]

The Windows failure seems unrelated, happens on master too.

[nirbheek]

LGTM, rebased to see if fixes from master fix the CI.