Ease ability of rules to test themselves with this library

[ittaiz]

Currently it's somewhat hard with this library to have rules test themselves since it's hard to "copy" rule contents and dependencies to inner test workspace.
One attempt was done by @or-shachar in an early incarnation of this PR.
The solution might lie not in this library itself but in having rules utilize buildozer scripts to add filegroups recursively (if so we should probably add docs about it)