closes#5473
Prior to this PR we were not exiting early for external packages, which
caused the `configure_options` property of the contexts to fail with
e.g. a key error because the DAG gets truncated for them. More
importantly Spack configure options don't make any sense for externals.
Now we exit early, and leave a message in the module file clarifying
that this package has been installed outside of Spack.
554937780b