This PR makes installAtEnd work as expected even
if maven-install-plugin is not used as extension.

How it works: it uses mojo Context to store "markers":
* presence of marker means project was "processed"
* value of marker tells what should be done (true = install, false = skipped)

UTs adjusted to provide plugin context (was null before).

Still, we MUST use String as these are spanning across projects,
hence due classloader, there is no single State enum.