Make sure that 'option java_package' always overrides 'package' in protobuf_gen

Found a bug where:

option java_package = "foo";
package bar;

was processed differently from

package bar;
option java_package = "foo";

For java code generation, the setting of 'option java_package' should always win.

Testing Done:
Added a unit test.

Bugs closed: 629

Reviewed at https://rbcommons.com/s/twitter/r/1108/
Sync of https://github.com/pantsbuild/pants/commit/1bdfef0cce41342938f249c88db2d23223827864