perfectly exact dependencies

[dominictarr]

opening this issue to discuss @defunctzombie's idea in #48 (comment)

I would like there be some way to specify an exact dependency,
but it has to be something that would be easy to port back to the npm registry, because I don't want to create another package management system, just reimplement a current one.

Oh, by the way, you can already create exact dependencies if you use a git url, git:github.com/USER/REPO#COMMIT

so, you can fork all your deps and convert them to that style, and you'll have immutable dependencies with the stock npm.