PathChooser: Trigger revalidation when environment or base name change

Trigger a re-evaluation of the state when the base filename or the
environment changes. Both can effect whether a command is found or
not.

Change-Id: I3fcb9683296113e04f7a78bb2d3ae681f6603823
Reviewed-by: Daniel Teske <daniel.teske@digia.com>
PathChooser: Small cleanup

Consistently use the same method to trigger re-evaluations.

Change-Id: I3af86ec8f811cfb64fcfd3784c8d3e79a8154b21
Reviewed-by: Daniel Teske <daniel.teske@digia.com>