Split the guts of pp_require into two static fns

Split the guts of pp_require into two static fns

S_require_version() and S_require_file() do the 'require 5.010001'
and 'require Foo::Bar' actions respectively.

This makes it clear that pp_require is effectively 2 disjoint functions,
and that all the local variables previously declared at the start of
pp_require actually belong exclusively to the file loading functionality.

This is based on a patch by Nicholas from 4 years ago, except that
I did the split from scratch since pp_require has been touched quite a
bit since then.

This commit splits it in such a way that the diff is kept as small as
possible. The next commit will re-indent.