Use PermissiveSlicer directly in tycho-build to compute project dependencies

[laeubi]

Currently we computer inter-project dependencies in an iterative manner, but actually it would be enough to do the following:

1. create a "project-iu" that has all requirements of the reactor project but make it optional
2. resolve this "project-iu" against all others excluding this using the PermissiveSlicer with includeOptionalDependencies=true and everythingGreedy=true and considerFilter=false

This should give us a good enough set of project dependecies to build the maven-project-tree.

[laeubi]

This is done now.