Export javac classes

Errorprone used to ship with its own copy of javac, but now just
uses the compiler directly. Errorprone plugins that use javac compiler
internal classes need to tell the compiler to export those classes.

Bug: 253827323
Test: m RUN_ERROR_PRONE=true javac-check
Change-Id: I282ea5a790839661f8fedb8cc79a182504118da0