Bug 8818: make sure we load modules before using them

Bug 8818: make sure we load modules before using them

An eval { eval "require $module;" }; was replaced with
eval { eval { require $module; }; }; which is a no-op, meaning that
the linker was not getting loaded, and the catalog module was throwing
up a big nasty error every time someone tried to save a record with a
heading. This patch replaces the require with can_load from
Module::Load::Conditional, which is PBP-friendly, and offers equivalent
functionality.

Signed-off-by: Nicole C. Engard <nengard@bywatersolutions.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>