Pass a PathMapper to the function that computes checksums

[kinow]

Closes #1828

I believe without the pathmapper we get a staged dir location that may not exist, resulting in the error reported in the issue. There is at least one other function in the visitors that take a pathmapper (I think the check_permissions?), so I did something similar to resolve the actual file so that we can access its stats when checking its hash.

@bartns if you could test this PR that'd be grand, but I'm still going to wait for someone else with more experience in this part of the code to review before adding tests (or fixing any test that may be broken after this change).

Cheers, Bruno

[bartns]

Hi Bruno, Seems to work! (Tested it only with example in #1828)