As a followup to #9197, this commit properly fixes the
exclude rule merging algorithm, by completely rewriting
it. The new merging algorithm works by implementing the
minimal set of algebra operations that make sense to
minimize computation durations. In order to do this,
this commit introduces a number of exclude specs
(found in their own package) and factories to create
actual implementation of those specs.
Specs represent the different kind of excludes we can
find:
 excluding a group
 excluding a module (no group defined)
 excluding a group+module
 excluding an artifact of a group+module
 patternmatching excludes
 unions of excludes
 intersections of excludes
With all those minimal bricks, factories are responsible
of generating consistent specs. The dumbest factory
will just generate new instances for everything. This
is the default factory.
Minimally, this factory has to be backed by an optimizing
factory, which will take care of handling special cases:
 union or intersection of a single spec
 union or intersection of 2 specs
 when one of them is null
 when both are equal
Then we have a factory which performs the minimal algebra
to minimize specs:
 unions of unions
 intersections of intersections
 union of a union and individual specs
 insection of an intersection and individual spec
 ...
This factory can be as smart as it can, but one must be
careful that it's worth it: some previously implemented
optimizations (like (A+B).A = A turned out to be costly
to detect, and didn't make it the final cut.
Yet another factory is there to reduce the memory footprint
and, as a side effect, make things faster by interning
the specs: equivalent specs are interned and indexed, which
allows us to optimize unions and intersections of specs.
Last but not least, a caching factory is there to avoid
recomputing the same intersections and unions of specs
when we have already done the job. This is efficient if
the underlying (delegate) specs are easily compared,
which is the case thanks to the interning factory.
All in all, the delegation chain allows us to make
the algorithm fast and hopefully reliable, while
making it easier to debug.

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