Use fallback from wrap file when force fallback

Optional dependency like below will now fallback to the subproject defined in the wrap file in the case wrap_mode is set to forcefallback or force_fallback_for contains the subproject.

# required is false because we could fallback to cc.find_library(), but in the
# forcefallback case this now configure the subproject.
dep = dependency('foo-1.0', required: false)
if not dep.found()
  dep = cc.find_library('foo', has_headers: 'foo.h')
endif

[wrap-file]
...
[provide]
dependency_names = foo-1.0