Stabilize support for friend clauses

[jcp19]

Friend clauses have been introduced in #810, but the heuristics used to match the packages mentioned in the friends clauses with the packages that receive the resources are still very finicky and config dependent, and they do not work in our test suit (although they work when running outside the test suite).

This issue keeps track of two tasks that need to be done before this feature is stable:

• make the heuristics to match the packages independent of the config
• add a check that friendsClauses do not create loops in forwarding resources (this is prevented by the checks for that import relations are acyclical - for a package to send packages to itself, it must be able to import itself and state permission preconditions. that is not possible, as cyclical imports are rejected by the compiler. This generalizes to chains of imports with more than one package.