feature.pm: Move hint normalisation to separate function

feature.pm: Move hint normalisation to separate function

PL_hints/$^H can hold feature bundle hints that cause %^H to be
ignored when features are looked up.

When feature->import and ->unimport are invoked, they set bits in $^H
such that %^H is used once more.  But they have to modify %^H to con-
tain what the bits in $^H imply.

Up till now, unimport was delegating to import, which meant that more
work was being done than necessary, because import would then detect
the special condition of $^H and repeat (some of) that work.