DERBY-6169: Reduce visibility of classes and methods under impl/sql

- Change visibility of all classes generated from sqlgrammar.jj to
  package protected.

- Make CompilerContextImpl.getParameterTypes() package protected since
  it's never called from outside the package.

git-svn-id: https://svn.apache.org/repos/asf/db/derby/code/trunk@1476915 13f79535-47bb-0310-9956-ffa450edef68