SI-7815 Dealias before deeming method type as dependent

To enable eta-expansion of method types seen from a prefix that
renders the result type as independent from the parameter symbols.

The enclosed test shows that we dealias types before checking
dependence, and that we do this deeply (e.g. type arguments are
also dealised.)

An existing test, neg/error_dependentMethodTpeConversionToFunction,
confirms that bona-fide dependent methods are still prohibited from
eta expansion.

Review by @adriaanm. I think this is sufficiently localised and easy enough for us to reason about to consider inclusion in 2.10.4, but if you prefer I'll retarget at master.