allow metadata to be embedded in source repos via .fdroid.yml

allow metadata to be embedded in source repos via .fdroid.yml

This allows a source repo to include a complete metadata file so that it
can be built directly in place using `fdroid build`.  If that app is then
included in fdroiddata, it will first load the source repo type and URL
from fdroiddata, then read .fdroid.yml if it exists, then include the rest
of the metadata as specified in fdroiddata, so that fdroiddata has
precedence over the metadata in the source code.

This lets `fdroid build` apps without having a whole fdroiddata setup, but
instead just directly in place in the source code.  This also lets devs
optionallu maintain the fdroid metadata as part of their app, rather than
in fdroiddata without loosing any control.  This should make it easier to
spread around the maintenance load.