Prevent callback from firing multiple times

[Nayni]

Took me a little to figure this out, but the solution was pretty simple actually.

When tsconfig-paths returned with an error the original code would invoke the callback but it would then fall-through to still try doResolve which would in turn invoke the callback again.

The different between Webpack 3 and Webpack 4 is that in version 3 they used async to wrap their callbacks but with version 4 this was replaced by neo-async which prevents you from calling the same callback multiple times.

I refactored it for both versions so that the callback is invoked and returned early. Knowing that tsconfig-paths already errored there is no reason to even try and resolve.

I also slightly changed both inner callbacks to be more consistent as I spotted a potential issue (I wrote some comments to explain).

Hopefully this fixes #11 (Although it will probably just show an error from tsconfig-paths now, but that should hopefully be solvable ;-))

[Nayni]

Please also check out: dividab/tsconfig-paths#29 it's probably related.

[jonaskello]

I merged and released dividab/tsconfig-paths#29 in tsconfig-paths 3.1.2. I will update the plugin to use that version after merging this too.

[jonaskello]

This is now released in 3.0.1.