SI-8755 Phase Assembly is broken

    - Fixes various bugs in the phase assembly of the Scala compiler (see below and JIRA ticket)
    - Plugins with empty after and rightAfter constraints are no longer ignored
    - runsRightAfter no longer causes other constraints to be ignored
    - constraint systems are now accepted iff they are solvable
    - No more illegal modifications of objects contained in hash sets (hash code no longer dependent on field values, identity semantics sufficient)
    - Realized constraint solving by well-known topological sort algorithm (due to Kahn (1962))
    - Fixes #8755
    - backport to 2.11.x