Using custom required: conditions based on msvc fallbacks means that
cairo can be found whenever it is installed, even when disabled. This
breaks logic checks later on, and produces wasteful lookups when force
disabled anyways.

Fix this by using the same logic meson would use with a true feature
option, and cause cairo to always be not-found when it is disabled,
using a guaranteed blank / not-found dependency.

Two of the three dependencies in question already used the feature
option in `required: ` and were thus guaranteed to be not-found when
disabled. The remaining one is cairo, which formerly failed to do that
until the previous commit.

meson: fix automagic dependency lookup for cairo

See merge request GNOME/gjs!917

We are currently falling between the cracks into a situation where the
responsibility of building the GLib GIRs is being transferred from
gobject-introspection to GLib. On the Docker image we have a g-i that is
new enough to require a newer GLib, which the build pulls in via a
subproject. However, building GIRs in GLib-as-a-subproject is not yet
supported, so the GIRs don't get built properly.

We solve this by building a new enough GLib and installing it on the
system, so it won't be pulled in as a subproject.

The non-debug image doesn't need to be updated because there we just use
the system's gobject-introspection.

CI: Build newer GLib in debug Docker image

See merge request GNOME/gjs!919

Same for GioUnix/GioWin32 APIs accessed through Gio. Any g_unix_ and
g_win32_ APIs should now be accessed through the GLibUnix/GLibWin32/
GioUnix/GioWin32 typelibs. The old way will still work, but we would
like to encourage people to update.

This check occurs in the namespace object's resolve handler. I don't
expect that to be a particularly hot path, so it's OK that we do a couple
of g_str_has_prefix(). If that path turns out to be hot, it could be
further optimized.

We also have to fix the gi.versions properties for GLibUnix, GioUnix,
GLibWin32, and GioWin32 to '2.0'.

Finally, we have to make sure that the GioUnix or GioWin32 typelib is
loaded before Gio, otherwise platform-specific types returned from
introspected functions will be looked up in Gio instead of the platform-
specific typelib.

See: GNOME/glib!3892

Closes: #599

Deprecate accessing GLibUnix/GLibWin32 APIs through GLib

Closes #599

See merge request GNOME/gjs!918