If a dep is not found on the system and a fallback is specified, we
have two cases:
1. Look for the dependency in a pre-initialized subproject
2. Initialize the subproject and look for the dependency
Both these require version comparing, ensuring the fetched variable
is a dependency, and printing a success message, erroring out, etc.
Now we share the relevant code instead of duplicating it. It already
diverged, so this is a good thing.
As a side-effect, we now log fallback dependencies in the same format
as system dependencies:
Dependency libva found: YES
Dependency libva found: YES (cached)
Dependency glib-2.0 from subproject subprojects/glib found: YES
Dependency glib-2.0 from subproject subprojects/glib found: YES (cached)
Xavier Claessens