Fixes how module names for messages with package prefixes are rendered.

- detected while prototyping with beefcake
- fixed on the internal beefcake fork here
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This change replicates that fix for the official code generator.

TODO: add a test; what's normal done to test features like this the proto
codebase? Add another test.proto/golden file?
	Change on 2014/12/11 by temiola <temiola@google.com>
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Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=81916292