imports: spack uses importlib instead of imp when available

- `imp` is deprecated and seems to have started having some weird
  issues on certain Linux versions.
  - In particular, the file argument to `load_source` is ignored on
    arch linux with Python 3.7.

- `imp` is the only way to do imports in 2.6, so we'll keep it around for
  now and use it if importlib won't work.

- `importlib` is the new import system, and it allows us to get
  lower-level access to the import implementation.

- This consolidates all import logic into `spack.util.imp`, and make it
  use `importlib` if it's avialable.

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