Better support for "installed" dependencies

[kentonv]

Ekam currently is very much designed to depend directly on the source code of dependencies and compile them as part of your own build. It is not very good at handling installed dependencies, especially installed dependencies that define their own Ekam rules, like Cap'n Proto.

(This is a very big project that I won't have time to work on any time soon.)

See sandstorm-io/capnproto#229

[zimbatm]

This makes packaging with other systems unnecessarily hard. The builds are also not reproducible since the git remote might change at any time. See https://github.com/sandstorm-io/sandstorm/issues/245