forced fallback doesn't work as documented for find_program

[matthiasclasen]

GTK has a gi-docgen subproject, and the wrap file has program_names = gi-docgen in its provides section.

GTK uses find_program('gi-docgen') in its meson.build.

I configure GTK with --force-fallback-for=gi-docgen.

meson doesn't use the subproject and continues to use /usr/bin/gi-docgen.

This is wrong. It should have built the subproject and used the gi-docgen binary that it provides.

[dcbaker]

Looking at the code there is no attempt to make that case work, we only honor the --force-fallback-for flag in cases of dependency, and not find_program.

I've had it in mind to refactor the internals of find_program to work more like dependency, so I'll see if I can solve this at the same time.