The idea of using reflection was so that a developer could
delete a package they didn't want and have everything else
still compile. However, a developer doing this is likely
building from source, in which case editing the factories
too is pretty trivial.
Removing the reflection makes specifying extractor flags
via the default factory easy, and removes the need for
special proguard config.
Issue: #2657
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=152810423
85adecf948