tests: each mock package now has its own class (#16157)

Packages in Spack are classes, and we need to be able to execute class
methods on mock packages.  The previous design used instances of a single
MockPackage class; this version gives each package its own class that can
spider depenencies.  This allows us to implement class methods like
`possible_dependencies()` on mock packages.

This design change moves mock package creation into the
`MockPackageMultiRepo`, and mock packages now *must* be created from a
repo.  This is required for us to mock `possible_dependencies()`, which
needs to be able to get dependency packages from the package repo.

Changes include:

* `MockPackage` is now `MockPackageBase`
* `MockPackageBase` instances must now be created with
  `MockPackageMultiRepo.add_package()`
* add `possible_dependencies()` method to `MockPackageBase`
* refactor tests to use new code structure
* move package mocking infrastructure into `spack.util.mock_package`,
  as it's becoming a more sophisticated class and it gets lots in `conftest.py`