sourceMapPathOverrides application order

[roblourens]

I think this is mentioned in one or more issues but I don't believe it has it's own issue yet.

sourceMapPathOverrides was previously applied by key order within the object which is unintuitive. I am changing it to be applied by the length of the key. This not perfect, and we lose the flexibility of being able to arbitrarily order the keys, but I think it is more likely to be correct. I'll try it out and see what breaks.

If we need to return to key order, it should be an array rather than an object.