Add automatic package registration and improved installation to Laravel Mix support

[LukeTowers]

Refs: #440, #401.

[LukeTowers]

@bennothommo with these changes all that's really needed from the package registration is the relative path to the winter.mix.js file from the base path. Do you see any value in continuing to have an array of values (path, package.json, winter.mix.js) tied to the package name instead of just the one relative path?

Personally I'm thinking not, even if we wanted to expand our dependency checking logic in the future to look at related package.json files it would be better to autogenerate the path to those files instead of have separate config items for it since supporting them being set values in an array opens us up to the possibility that they could not match the mixJs path at some point in the future.

[bennothommo]

@LukeTowers I started writing a PHP class to handle package.json manipulation for my wip/snowboard-workspaces branch here: e33fa94

Would that be at all useful for this? Might be able to move some of that procedural code into this class.

[LukeTowers]

@bennothommo I was looking at that but I didn't really see the point to be honest. It felt a bit too much "enterprisey" without a clear reason for its existence or usefulness. Could you provide an example of how we could improve this code by using that class and / or what else that class could be used for?