Add support for describing symlinks in Git trees

[mih]

Likely not for annexed files (see #50), but for other symlinks pointing inside a dataset (but not into its .git).

[mih]

This would be a specific type of a Distribution. It could be modeled on a technical level, which the content of the distribution being a symlink that is "available for download" somewhere. In this case, there is no need to support anything in addition.

Alternatively, it could be modeled on a semantic level, e.g. with such a distribution being same_as another distribution (within the same parent distribution).