Fix source maps caching key

[Strech]

Closes #344

Add processed source file dependencies to the source map.
This asset call @env['application.js.map'] will add compiled application.js dependencies with all derectives to the application.js.map asset

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[Strech]

A bit about my investigation:

I was trying to make it in other way - through normal pipeline process, but I've failed. When we are requesting application.js.map and if we have application.coffee source file, the mime type system will recognise it as type = "application/js-sourcemap+json" and file_type = text/coffeescript.

And the pipeline will looks like that: [SourceMapProcessor, DefaultSourceMap, DirectiveProcessor, CoffeeScriptProcessor]

So this mean that if I have require directive inside my application.coffee file, it will fail to find this require statement file, because the initial type which will be passed as accept option to resolve method in DirectiveProcessor will be application/js-sourcemap+json. And basicaly this is the main issue - type mismatch.

We should have application/javascript type for all the processors, exept SourceMapProcessor. But I can't find a simple solution for that. So I decide that it's better to keep all the ugly code in one place, to be sure that it will not speding out everywhere and in case of a better folution we can easily wipe it out.

Maybe if you have any suggestions to implement it better way, I would be glad to hear.
Thanks!

[Strech]

@rafaelfranca If you have a time, can you 👀 on this PR?

[schneems]

I was able to repro with the linked example app, and I confirm your patch fixes it.

A source map should depend on the same things it's parent's dependencies. Thanks a ton for your help and this patch.

[Strech]

@schneems No problem, I want to help your team more 👍