feat: add a yalc-using package

[aleclarson]

During yalc publish, the local .yalc directory is now copied to the global store, but its files are excluded from the content hash. Finally, the local yalc.lock file is copied and hashed.

The relevant commit: a9263e0

(Probably wanta merge #66 first)

[aleclarson]

Note: If you used yalc link in a package, the published version of that package will not use the linked package, because yalc link doesn't affect the package.json file.

[aleclarson]

We need to consider if yalc push should be cascading or not. For example, if I do yalc push and it updates another yalc-using package, should that package be pushed as well? I'd say we probably want to prompt the user for them to decide. 😄

[aleclarson]

TODO: Skip copying .yalc/{name} directories that correspond to devDependencies.

[aleclarson]

Another thing to consider:

Since node.js doesn't dedupe packages based on version, there's a good chance of having multiple "instances" of the same package when yalc is used extensively in a dependency tree. This can easily lead to confusing bugs.

We could prevent duplicate yalc-linked packages by hoisting them or by linking to the same .yalc directory for every yalc-using package; neither of which sounds particularly easy.