Revert wrong optimization in 1b83945. Always reset compilers when the
classpath has changed and is valid.

Previously we tried to only reset compilers on a transition from an invalid
to a valid class path. That’s wrong, since a change in a valid class path
still needs a reset (the compiler can’t refresh its symbol table any other
way). This lead to spurious errors.
Merge pull request #722 from dragos/issue/wrong-optimization

Revert wrong optimization in 1b83945. Always reset compilers on classpath change