Duplicate dependencies for async modules with async functions

[steverep]

When using the runtime option, the plugin requires the @babel/runtime helpers outside the async function wrapper. However, babel-loader may have already added the same imports if the module also contains a transpiled async function. The compilation could potentially be better optimized by eliminating the inner dependencies in favor of the outer ones.

Note this should, in theory, be a fairly minimal optimization. This is because the current situation just results in loading the module outside the wrapper, then assigning the same cached module to a different variable.