Remove GNU findutils and coreutils dependenices

[mikepqr]

It should be possible to remove these dependencies so stowsh can run on BSD systems (including macOS). See #13.

I don't personally have a need to fix this issue, but contributions welcome.

Some difficulties:

• we need a way to construct the relative path from an existing file to a non-existing file. We currently do this with realpath foo --relative-to=bar --canonicalize-missing. This script may work, but introduces a readlink dependency. (BSD readlink doesn't do the right thing for our use case.)
• BSD find complains on uninstall. Have not investigated why.

[mikepqr]

Could use that script with this for realpath.

[mikepqr]

Consider what to do if a package contains a symlink (say ~/.dotfiles/vim -> /usr/bin/vim). Should ~/vim point to ~/.dotfiles/vim or /usr/bin/vim.

Write tests that assert whatever behavior is chosen.

[mikepqr]

https://github.com/bashup/realpaths