Allow packages to inherit?

[alecthomas]

There are a couple of cases where various approaches to this would be useful:

A conservative approach might be to just allow versions to be merged, though this could get confusing with how Hermit inherits attributes from parent scopes.

A potentially more complex approach would be to merge configs. There are a couple of places where this would be useful for Square:

• Our internal Go manifest replicates the public one with the minor addition of GOPRIVATE being set.
• Our internal OpenJDK manifest is identical to the public one except for importing our private CA.