Implement symlink and tree install for pkg_install

[kgreenek]

Implement _install_treeartifact and _install_symlink functions or the pkg_install rule.

[jacky8hyf]

Apologies, I have made a similar contribution for TreeArtifact in https://github.com/bazelbuild/rules_pkg/pull/885 , which causes a merge conflict for this pull request. I am happy to see someone is also working on the symlinks, though!

[aiuto]

I am closing this as a duplicate of #885

[kgreenek]

This is partially a duplicate, but my PR also added support for symlinks, in addition to the TreeArtifact implementation.

I'll rework mine when I have time and send a new PR with just that functionality.