Use realpath instead of readlink in binary wrappers

Created by: aarongable

Readlink directly returns the target of a symlink. If the symlink is relative, readlink only returns the relative path. This path is relative to the buck executable on $PATH, not to the current working directory, so the Python invocation fails.

Using realpath instead directly resolves the full absolute path referred to by the symlink, allowing users to use relative symlinks in their bin/ directory on $PATH.