Java: Add Automatic-Module-Name entries to the Manifest

[ennerf]

This PR adds Automatic-Module-Name entries to the Manifest files. This allows Java9+ users to depend on protobuf, without having any impact for users of earlier versions.

The module names match the OSGI symbolic names as recommended in JLBP-20

This fixes #3903 and #6493

[elharo]

I think this one should go into the release notes.

[elharo]

As @netdpb suggested, the question here is the relation ship between the lite and regular artifacts. If both can reasonably appear in a classpath at the same time this won't work. Anyone know the distinction between these two?

Since lite is mostly for android which doesn't use Java 9+, it might make sense to drop the lite/pom.xml from this PR until we're sure.

[ennerf]

At the moment the lite version is a subset of the regular artifact. Considering that both artifacts use the same package and filenames they are guaranteed to always conflict, and users should really avoid having both of them in the same project.

For example, a regular and a lite runtime with different versions may end up loading a shared class like com.google.protobuf.Utf8 that is incompatible with the other. (At the moment it looks like pretty much everything is shared because all messages extend AbstractMessageLite)

The worst case would be if a dependency transitively adds the lite-runtime, but in that case users could manually exclude it. Any generated lite-messages would still run fine against the regular runtime.

Personally, I'd actually prefer to get a compile time error rather than potentially running into obscure issues at runtime.

[elharo]

FWIW in Java 9 and later you will get a compile time error if both of these appear in the classpath.