[MNG-8006] SPI to contribute to effective properties and more

[cstamas]

Create aether internal "extenders" components to serve several purposes:

• extender to meddle with exec request, for example to mediate between effective properties SPI (exec request extender)
• extender to meddle with prepped session and related bits, for example to move out deprecated code from session factory (session extender)

This PR implements M4 API SPI for contributing effective properties and exposes method for effective properties on Session.

• Session should expose properties service "ready" for interpolation, otherwise each interpolating code (plugin, etc) would need to construct the "effective" ones themselves, repeating same steps over and over again.
• SPI can contribute to user properties
• Session exposed properties are all immutable (sans effective, that is computed on each call).
• and lastly, a cleanup: all packages within o.a.m.internal.aether are now package protected, to tighten internal encapsulation. New core package o.a.m.resolver introduced that contains 2 publicly accessible classes.

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https://issues.apache.org/jira/browse/MNG-8006
https://issues.apache.org/jira/browse/MNG-8021

[cstamas]

Fixing the UT (changed the source of props) is now beyond my current capabls 😄 but @gnodet i think you see what the goal is: have some internal stuff that mediates.

[gnodet]

I think we need an IT. I have some doubts on the fact that properties set by an EffectivePropertyContributor will end up in the maven session and made available for project interpolation...
They are currently set in the repository session's configProperties, but I don't think they are pushed up to user properties and in the maven session.