docs: update old docs for `spack.package.PackageBase`

- There was a lot of documentation in `PackageBase` dating back to the
  very first versions of Spack.

- It was repetitive and out of date, and the docs at spack.readthedocs.io
  are better.

- Remove the outdated specifics, and leave the minimal useful set of
  developer docs in `package.py`.