Deprecate the app_engine sub-block of google_project, and create a
google_app_engine_application resource instead. Also, add some tests for
its behaviour, as well as some documentation for it.
Note that this is largely an implementation of the ideas discussed in
#2118, except we're not using CustomizeDiff to reject deletions without
our special flag set, because CustomizeDiff apparently doesn't run on
Delete. Who knew? This leaves us rejecting the deletion at apply time,
which is less than ideal, but the only other option I see is to silently
not delete the resource, and that's... not ideal, either.
This also stops the app_engine sub-block on google_project from forcing
new when it's removed, and sets it to computed, so users can safely move
from using the sub-block to using the resource without state surgery or
deleting their entire project. This does mean it's impossible to delete
an App Engine application from a sub-block now, but seeing as that was
the same situation before, and we just papered over it by making the
project recreate itself in that situation, and people Were Not Fans of
that, I'm considering that an acceptable casualty.
