rationalize directory loading, directory traversal, and --force

[ctb]

we can force sourmash commands to load .sig/.sig.gz files from under a directory, and (with -f) we can ask sourmash to try to load other files that MIGHT be JSON files. But that doesn't try to load other sourmash-loadable files like .zip files and so on.

we should also look at how this all intersects with pathlists, which have other issues; see #1845

there's also some wonky stuff with -f and zip files and manifests, see #1849

[ctb]

kind of relevant: require .sig or .sig.gz extensions on signature JSON files #1932 ?