MNG-7400 - Allow more WorkspaceReader's to participate

[laeubi]

We at Tycho have a high demand in integrating better with maven to reduce some workarounds that where used in the past to get custom dependency types into the maven dependency model (namely using system scoped absolute paths).

For that reason it would help if it where possible to register additional WorkspaceReader to participate.

I have created a prototype to show/test this new feature.

FYI @mickaelistria @akurtakov

This PR allows maven-extensions to supply WorkspaceReader as it is possible with AbstractMavenLifecycleParticipant currently it is only possible with some workarounds:

1. One needs to register a AbstractMavenLifecycleParticipant in a maven-core-extension
2. In the afterSessionStart cast the RepositorySystemSession to DefaultRepositorySystemSession
3. Call setWorkspaceReader and take care not to replace an already exiting one

With this change it is possible to simply register WorkspaceReader component directly and Maven takes care of all the rest.

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[mthmulders]

Thanks a lot for contributing.

I'm not done yet with the review (I just need a bigger screen which I don't have available at this moment); I'd like to take a more thorough look soon.

In the meantime, would you be so kind to revert the unrelated change in imports? It allows us to focus on the actual changes you're proposing. Thanks!

[laeubi]

I'm not done yet with the review (I just need a bigger screen which I don't have available at this moment); I'd like to take a more thorough look soon.

Thanks for looking into this, there is currently an issue that this also finds the org.apache.maven.ReactorReader I'm currently try to find out how to avoid this as this obviously is not what one wants :-/

I'll try to cleanup the unrelated parts, it just seems the Eclipse-Formater likes to format more than desired here...

[laeubi]

In the meantime, would you be so kind to revert the unrelated change in imports? It allows us to focus on the actual changes you're proposing. Thanks!

Done! mvn clean verify also runs without test or checkstyle errors now!

[laeubi]

I have uploaded my "HelloWorld" here: https://github.com/eclipse/tycho/pull/586/files

[mthmulders]

Thanks for your patience. I was able to have a better look now and I left a couple of questions.

[michael-o]

@laeubi Please address the open comments before I am merge this.