Move rules_pkg targets into //pkg, and add experimental C++ library rules

[dlj-NaN]

This change moves the pkg_* rules into the //pkg package, which cleans up the root package.

It also adds an experimental cc_dist_library rule, which is similar to Bazel's cc_import rule. The goal of cc_dist_library is to produce output libraries from several targets. For example, splitting //:protobuf into multiple targets means that bazel-bin/libprotobuf.a won't contain all of the objects. The cc_dist_library creates a single library from several different cc_library targets. This may be useful for future packaging targets.