Give full control over manifest

[epage]

The current merge strategy is too simple.

[epage]

From the code

Note that the "merge" in this case is relatively simple: only top-level tables are actually merged; everything else is just outright replaced.

We might want to merge the package tables rather than having one completely overwrite the other. ie we might want to recurse on some tables but not fully.

[epage]

In #140, we moved from merging manifests to overwriting the specific fields we are interested in, bypassing this question