Verify hard links before use (#61)

Move existing file verification to after the hard links are created to
avoid redundant verification of local files while other processes are
downloading other files, and to ensure the file actually being used by
the classloader has been verified.

This also moves the staging of the resources from the loader function's
parameter Supplier to the loader function (LccUtils.createClassLoader),
which simplifies the code slightly.

This fixes #59