Default sourceDirectory forces imports from index

https://github.com/Andarist/lerna-alias/blob/e865ecb3d12f262a3e17b60af9f24aa717ecf339/lib/index.js#L35 causes it to be the case that if I want to import from ```myPackage/src/notIndexFile.js``` this is impossible; I must export everything from the index and import directly from there. if we did not include index in the string, I believe everything would still work that wants to import from index by default, but it would also be possible to import from other files and folders in the directory.