Previously, when someone activated rbenv shim `foo`, rbenv would error out if `RBENV_ROOT/versions/RBENV_VERSION/bin/foo` did not exist.

This change allows `foo` to be additionally looked up in PATH as fallback. This prevents a case where one of the Ruby versions is shadowing a system-wide command of the same name, preventing the use of the global command across all other Ruby versions.

rbenv used to be strict around this, allowing the fallback to avoid ever falling back to system Ruby for invocations like `bundle` if the current Ruby version did not yet install Bundler. The current approach prevents this scenario by explicitly disallowing fallback for the following executables: ruby, rake, gem, bundle, bundler, irb, rdoc, ri.