Dependency parsing should have at most one "/"

[sitch]

Bug Description

Currently something like this:

// package.json
{
  prettier: "@myrepo/prettier/profiles/recommended"
}

Will fail using:

$ npx depcheck --specials=prettier

This is because the actual string "@myrepo/prettier/profiles/recommended" is treated as a package instead of stripping the trailing path into "@myrepo/prettier".

This seems to be consistent across all specials I've used and not just limited to using the prettier special.

The fix for this is just to add a helper function to strip the trailing path portion of a possible dependency string.