Porting/checkURL.pl now requires rather than uses all non-core modules.

Porting/checkURL.pl now requires rather than uses all non-core modules.

This way ./perl -Ilib -c Porting/checkURL.pl can be used to syntax check it.

Only File::Slurp and URI::Find::Simple were actually relying on C<use> to
import subroutines - replace the two uses with fully qualified names.
All other packages are needed for object constructors, not imports, so there
is no change in loading them with C<require>.