Add support for depending on local Hex packages

[ericmj]

Offline environments or packages built with mix hex.build should be able to be depended on directly.

Proposal for package definition: {:ecto, "~> 2.0", hex_local: "foo/ecto-2.0.2.tar"}

Notes for implementor:
This dependency should not be handled by the remote converger because running dependency resolution for it does not make any sense. It should also probably be a separate Mix.SCM implementation.

/cc @josevalim

[wojtekmach]

we should probably also have a task to manually download a package (which was recently mentioned on Slack), e.g.:

mix hex.download ecto --to vendor/hex # creates vendor/hex/ecto-2.2.6.tar
mix hex.download ecto 2.2.6 --to vendor/hex # creates vendor/hex/ecto-2.2.6.tar

and a task similar to mix hex.vendor copy mentioned in #304 (comment)

[ericmj]

Yeah, after we implement local packages we can add mix hex.download and then add the vendor tasks if we feel they are needed.

[josevalim]

My main interest in such a task was for testing but with the smoke stuff now in hex.build I am probably happy with the way things are.

[ericmj]

I agree there's less need for this feature now, but it would still be a nice thing to have. Closing this but if someone wants to contribute it feel free of course.

We need a download task because it's difficult for users to manually download private packages today since you can't just wget the URL.