Enter async mode only when necessary

[jscheid]

Hi,

Ever since babel-loader introduced caching we can't use it anymore together with enhanced-require as that doesn't support async callbacks in loaders.

This change restores enhanced-require compatibility as long as the cacheDirectory option isn't used.

(It would be possible to implement caching synchronously but I suppose there are performance concerns. We don't need caching when using enhanced-require so I left caching to be async.)

[jscheid]

Whoops, totally missed #60! Sorry about that. I guess @camshaft and I have the same problem and approached it from different angles. I don't really care which of the two PRs gets merged, it looks like both will do the trick so feel free to close this.

[dashed]

This PR seems more simpler than the other one.

[camshaft]

the other one does support sync caching though. i could go either way as well.

[Couto]

The problem with the other one, was that the PR had bad timing since I was rewriting a lot of stuff.
I'm not familiar enough with enhanced-require so I do have to ask... Does caching provide a benefit?

[jscheid]

How about merging this one then, seeing that it's pretty straightforward, and leaving the other one for when somebody finds time to review it?

[Couto]

@jscheid I think that's a good point, and sorry for taking so long on this matter.
I've merged the PR with the develop branch, can you give a try on some project? If you find no errors with it, I'll make a release with this PR

[jscheid]

@Couto thanks, will do -- switching over from our fork to your develop now. I'll let you know how it goes.

[jscheid]

@Couto seems to work fine!

[Couto]

I've merged this. You can find it on npm with version 5.3.0
Thanks everyone. If you find any error, please open an issue :)

I'm not familiar with enhaced-required, if caching is something that benefits its process, please tell me, so that I can open an issue to rework the first PR.