Add a rule to materialize a set of pkg_* instances.

The concept is that
- you organize your desired file tree with a set of `pkg_*` rules
- they are input to `pkg_materialize(name='foo', srcs=[...])`
- this creates a symlink tree within the calling package to the artifacts represented by `srcs`.
- Finally, you can use `foo` as a `data` input to a `*_binary` rule.

This might be a decent solution to the silliness of bazel's runfiles layout. You would define a tree that places everything under 'usr/share/my_app/', then write the binary for 'my_app` to expect files in /usr/share/my_app/..... Add a pkg_symlink in the final archive pointing from /usr/local/bin/my_app to /usr/share/my_app/bin/my_app and you've got reasonable tooling.

Of course, this only handles `data` dependencies which were explicit in the first place. It does not deal with the generic runfiles problems like .so and .py files.