[feature request] Infer Java package from soy namespace in SoyParseInfoGenerator

It would be great if `SoyParseInfoGenerator` had an option to infer the Java package from the soy namespace of the file.

See https://github.com/closure-tools/closure-templates/commit/7acea66c65ee0d7b4ceab2d18247cf8d8f15a437 for a prototype.

/cc @robfig @sgammon

Start by locating SoyParseInfoGenerator and reading the prototype commit linked in the issue. Compare that behavior with the generator's current package handling, then verify that the requested option can infer the Java package from a Soy namespace and preserve existing behavior when it is not enabled.

Written by the indexing model from the issue text.