(maint) clean up imports
(maint) delete errant ':else' in 'condp'

This was syntactically incorrect; 'condp' simply uses the last form passed to it
as a default if none of the other forms are matched.
(maint) add schema validation fixture to all tests

This commit turns on schema validation in all test namespaces via the
standard fixture.

As a result of this, it was also necessary to add additional error
handling in 'hanlde-prismatic-exception!' for the case when schema eagerly
validates the service graph.  In this case, a different exception is
thrown from schema, and it requires a bit more tomfoolery to extract the
list of missing services from it.

An alternative solution to this problem would be to do a manual
validation before instantiating the graph, instead of relying on schema
to throw this exception ... although I haven't yet been able to figure
out how to do that.
fix indentation
delete old, irrelevant comment
WIP: stubbed out required-config function
(maint) delete unused def
WIP