Allow more WorkspaceReaders to participate

Currently there are two kind of WorkspaceReaders the reactor one and the one obtained from the RepositorySystemSession (which is eed by an IDE scoped component in org.apache.maven.internal.aether.DefaultRepositorySystemSessionFactory).

In Tycho we have the demand to supply a WorkspaceReader as there is a so called "target platform" as the source of artifacts.

Now it does not seem correct to declare it as an IDE, and of course we don't want to block the IDE use-case.

Another issue is that at the time the IDE component is injected there is no session and thus a session scoped component won't work as well so even if we would use this we are rather "blind".

It would be good to have maven discover WorkspaceReaders as it discovers AbstractMavenLifecycleParticipants in the following way, right before the session is fixed:

for ( WorkspaceReader reader : getWorkspaceReader( session.getProjects() ) )
{
	//chain them together with the current workspace reader(s)
}