This updates PlasticClient to add formal support for dependency

checks. PlasticClient.check_dependencies() now determines if cm is

installed.
PlasticClient.get_local_path() now calls this instead of checking for

dependencies itself, emitting a warning if dependencies haven't already

been checked via setup() or has_dependencies().
Unit tests for Plastic didn't previously exist. This change adds some

dependency check tests for it.
It's unclear whether Plastic still actually works today, unfortunately,

as this was a contribution that never received updates. However this is

at least a step toward actual testing.

Unit tests pass on Python 3.7-3.11.