Cache directory relative to executable path

I currently have the problem that I want to move my cache directory relative to the executable path (sys.argv[0]).

This is needed because I want to have a shiv package distributed with an installer and all the package data at the installation directory. If the cache is global (e.g. ~/.shiv) I cannot remove it easily when deinstalling the package. Currently, I can only set the path to a directory relative to the current working directory or an environment variable. This is not feasible for me because the cwd can change and creating an environment variable would be a big overhead if installing multiple versions.

It would be great if I could specify something like "--root $__pkg__/cache" (introduce a special variable) or change the behavior to resolve "--root ./cache" relative to the sys.argv[0] directory. The first solution would also enable that you can use a subdirectory of a path set by a normal environment variable.

Start by tracing the existing --root handling and how paths are resolved relative to the current working directory or environment variables. Compare the two requested approaches around sys.argv[0], then confirm the chosen behavior supports installed shiv packages and allows their cache to be removed with the installation.

Written by the indexing model from the issue text.