Rework exclude rule merging

As a follow-up 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

- pattern-matching 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.