Considerations:
- `./libexec/rbenv` executable is the entrypoint to the program;
- BASH_SOURCE might be the path to a symlink that has activated `./libexec/rbenv`;
- We must resolve the symlink to learn where rbenv's libexec directory is;
- It's not guaranteed that rbenv commands will always remain directly under their own "libexec" directory, since a package maintainer can change that, e.g. rbenv commands are sometimes placed into `/usr/libexec/rbenv/*`;
- Resolving symlinks might fail and in that case we just assume rbenv project layout.
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