Fixing ProjectReload with missing artifacts

[sdedic]

It was discovered that if ProjectReload.withProjectState is used and the project contains a truly unresolvable artifact (i.e. a reference to a local custom build, that can be never downloaded), the priming support will keep telling the project state is not good. The past priming information (with an old timestamp) was incorrectly cached and has been merged with the new MavenProject, but degraded its timestamp ... so it was older than constituent files, which resulted in an inconsistent state right after load.

This PR extends the SPI to indicate that the ReloadImplementation should execute whenever previous ReloadImplementations changed anything. It also fixes various conditions with missing artifacts and project (re)loads - see the added test cases.