Support creating symbolic links to dynamic destinations

There is a general interest in supporting dynamic filesystem locations based on values such as versions, and platform OSes and CPU architectures. In #312, we discuss implementing this for the pkg_filegroup framework.

Users may want to create symbolic links to these dynamically-named destinations. To do this, they either need to share a template, or either have or be directly provided with information via a label. Template sharing should be avoided, as this can place an additional maintenance burden on users.

The other options potentially come with complexity of actually calculating the desired destination without arbitrarily restricting the user. Such restrictions should be avoided if possible, but it may not be avoidable to do so w/o creating additional complexity.

Some potential options and pitfalls are discussed in https://github.com/bazelbuild/rules_pkg/pull/293#discussion_r584810551, specifically [this comment](https://github.com/bazelbuild/rules_pkg/pull/293#discussion_r586572986).