Fix #17405 by not checking imported equations

Previously, we checked all imported type family equations
for injectivity. This is very silly. Now, we check only
for conflicts.

Before I could even imagine doing the fix, I needed to untangle
several functions that were (in my opinion) overly complicated.
It's still not quite as perfect as I'd like, but it's good enough
for now.

Test case: typecheck/should_compile/T17405