Add a User Local Cache

[coderoshi]

WIP

This adds a local cache onto rebar, which is a directory (~/.rebar/cache) where dependencies are housed. When a project builds

TODO:

• Add a configuration to turn this on. Make it off by default
• After N timeouts, set a remote source (ie. github) as "unreachable", and give up on updates, and rely entirely on cache. This allows continued development when github goes down. Which it will. Again.
• Separate the steps between pulling repo information and copying deps into a local project. This allows an analyze step, to build a dependency graph for analysis, namely: dependency cycles, version collisions, and pruning. With a graph, adding a dependency override option also become easier.
• Currently, a dep's source is updated on every build. This will continue without a version freeze/lock. Sadface. Build that into rebar.
• There's a danger of name collision, since rebar has no concept of groups or namespaces. Either add groups to rebar, or extract an optional group name from the repo (eg. basho).

[sinasamavati]

+1

I noticed that it doesn't use the publisher name which is so important.
What I'm doing in Mad is putting repositories in ~/.mad/repos/[publisher]/[repo] just to make it able to cache forks of a repository. For example, perhaps, I wanna use my fork of X in a project, whereas, someday in the future I'll use other projects that are based on the original version of X, so what's gonna happen is that rebar gets my own fork of X even for those projects that need the original version.