Replace deprecated functions with their direct replacements:
- dep.get_pkgconfig_variable() → dep.get_variable()
- prg.path() → prg.full_path()
- source/build_root() → project_source/build_root()
In one case we need meson.global_source_root() that was only
added in meson 0.58, so bump the requirement to that.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2077>
The Extensions app code is now independent enough from the rest of the code
base to move it to its own subprojects, like we did for the extensions-tool.
This allows for stand-alone builds of the app, which we are about to use
for distributing it as flatpak.
https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1133
