Change EquivalentContentResolver interface to take app config not srcs

Instead of taking a set of enabled sources it takes an
ApplicationConfiguration from which the former, and other info can be
derived.
add resolveAliases method to EquivalentContentResolver
EquivalentContent no longer sets equivalents on Content passed in

Since we can't guarantee that the content passed in for a given key is
the entire equivalence set we can't validly set the equivalent-tos. It
needs to be done prior to creating the EquivalentContent with
visibility of the full equivalence set.
Merge pull request #167 from atlasapi/mbst-7547-equiv-single-item

Change EquivalentContentResolver interface to take app config not srcs