This updates MercurialClient to add formal support for dependency

checks. MercurialClient.check_dependencies() now determines if hg

is installed.
MercurialClient.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 have been updated to include new dependency-related tests. If

hg isn't installed, tests that depend on it will be automatically

skipped.

Unit tests pass on Python 3.7-3.11.
Tested with and without hg installed.