This encapsulates the logic for turning an acceptable plugin name into
the absolute path for that plugin. It can be used to preprocess a
plugins list to map each plugin to its absolute path, which is necessary
if `babel.transform` is going to be executed on a file outside the
directory subtree where the plugins are installed.
This adds a getPossiblePluginNames helper encapsulating the logic for
what plugin names we should try to resolve, and the resolvePlugin method
just calls this helper and actually resolves them.
