Support additional fastlane metadata locations

After a discussion with @Bubu in matrix, we agreed on the following (A leading / to indicates root of the repo, not the file system):

Fastlane currently supports:

• /metadata/
• /fastlane/metadata/android/
• /<module>/fastlane/metadata/android/

F-Droid currently supports:

• The first two above
• /src/<buildFlavor>/fastlane/metadata/android/

And I will be adding support for

• The third form Fastlane supports, with behavior identical to the first two.
• /<module>/src/<buildFlavor>/fastlane/metadata/android/, with behavior identical to the extra version F-Droid supports.