index.xml cannot handle APKs with the same packageName/versionCode

index.xml cannot handle APKs with the same packageName/versionCode

Really, it is the fdroidclient parser of index.xml that fails, due to the
hardcoded expectation that there will only ever be a single APK for any
given versionCode.  We keep index.xml backwards compatible for old
clients, and use index-v1.json to support new things.  Having multiple
APKs that have the same packageName and versionCode will break the client
v0.103.* since that version uses index-v1.json, but still has the hard-
coded database parsing stuff.

#153