Fix require’s get-magic handling for @INC elements

It was only calling get-magic before checking whether the argument was
a reference if the array was tied, which is not the only thing that
can cause an @INC element to have get-magic.  It should have been
checking for get-magic on the element itself (which is a faster
check, too).

And then there were too many FETCH calls.

I do not know whether we should be calling get-magic exactly once
when the ‘Can’t locate’ error occurs.  At least this commit reduces
the number of FETCHes.
perldiag: Use (F) consistently

and explain in the prose when there is an error.  One entry already
explained this.
perldiag: Rewrap an entry for better splain output