Use realpath of scripts to determine relative locations (#308)
Just using paths relative to ${BASH_SOURCE} to access `libexec`
fails if the scripts are symlinked to a location,
such as when installed with Homebrew,
since the `libexec` directory is not linked into `brew --prefix`.
Implement a simple version of `realpath`
and run `${BASH_SOURCE}` through it.
This gets the actual installation directory
relative to which `libexec` is located.
Co-authored-by: Andrew Christianson <achristianson@edmunds.com>