Adding a vapi to 'sources' promote the depending library to a Vala target

[arteymix]

Here's the typical use case where you create some library and bundle a vapi along, potentially from vapigen, and use it along a C target:

foo_dep = declare_dependency(link_with: foo_lib, sources: foo_vapi)
bar = executable('bar', 'bar.c', dependency: foo_dep)

This is causing the following error:

Vala library has no Vala source files.

I'm either doing this wrong or having a *.vapi as source should not be a sufficient criteria to promote a target.

[nirbheek]

You're right, this is a bug. However, this is partly because the way we declare a Vala dependency is hacky. The correct fix is to make declare_dependency understand Vala dependencies. We will need this for Rust too.