Remove depth from find invocation

[Bouzomgi]

Shelm currently uses the depth argument in a find call. This argument exists in the macOS implementation of find, but does not in Linux implementations. This PR replaces this argument with a filter of all paths removing any that do not include 3 slashes, equivalent to the "--depth 3" argument that currently exists. Because grep will fail if this filter does not return a result, I've added a fallback in || : to ensure the function returns nothing if no directories are found, just like the existing script.

Basically, this change allows shelm to work on both macOS and Linux.

I've tested this implementation on macOS and an Alpine image, and fetch and make both work the same as how the code exists currently.

[vjousse]

Replacing -depth 3 with -maxdepth 3 -mindepth 3 does the trick for me on Linux (and it seems that those parameters exist on macOS).