Don't try to compute delta for projects neither existing in the
workspace nor in any of the build trees.
ResourceDeltaFactory.computeDelta() can't deal with that and will fail
with ObjectNotFoundException, breaking build.

Application use case: importing an Xtext based project that references
another (not yet imported) project via .project file. Xtext builder
reports it is interested in the project that isn't there (but is
referenced in the .project file).

Fixes https://github.com/eclipse-platform/eclipse.platform/issues/243