External packages are now registered in the DB (#1167)

* treats correctly a change from `explicit=False` to `explicit=True` in an external package DB entry.
* added unit tests
* fixed issues raised by @tgamblin . In particular the PR is no more hash-changing for packages that are not external.
* added a test to check correctness of a spec/yaml round-trip for things that involve an external
* Don't find external module path at each step of concretization
    * it's not necessary.. The paths are retrieved at the end of concretizaion
* Don't find replacements for external packages.
* Test root of the DAG if external
    * No reason not to test if the root of the DAG is external when external
packages are now first class citizens!
* Create `external` property for Spec (for external_path and external_module)
* Allow users to specify external package paths relative to spack
    * Canonicalize external package paths so that users may specify their
locations relative to spack's directory.
* Update tests to use new external_path and external properly.
* skip license hooks on external