Create pkg_tree rule for creation of in-buildroot directory structures

Sometimes, users need to pass directories to commands as a part of their builds. When these directories are created from many parts of the build tree, it can be convenient to use the `pkg_filegroup` framework to assemble them.

`pkg_install` is a bit raw for use internally, and may not be compatible in all circumstances. Among other things, it may assume that it can do things that aren't supported by or propagated out of the buildroot.

Consider instead a `pkg_tree` rule, which creates a tree of files rooted in bazel's buildroot, only doing the basics. The output could then be consumed by other targets as needed.