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-8006https://issues.apache.org/jira/browse/MNG-8021
The IT associated with this PR is using the Maven model to generate a hocon POM parser. This requires the maven-api-model module to attach the POM as an artifact, and the maven.yml change so that the model is present in the local repository.
