Dependency collisions (co-existence of multiple versions of the same assembly)

[eladb]

Various host languages have varying degrees of support for co-existence of multiple versions of the same module within their runtime closure. For example, node/npm simply allows that (since the runtime closure is a directory tree), Java supports it only with certain class loaders, etc.

The questions are:

1. Should the jsii-runtime support it (at the moment, it intentionally blocks this from happening)
2. What happens in environments where this is not supported? Do we have a way to help users resolve these conflicts?

[RomainMuller]

I think the compiler has opportunities to catch this problem, too. If jsii-runtime will not allow something, the compiler shouldn't allow it either (but they must still both validate, since what is built against may differ from what is run against).

[eladb]

We determined that the best approach for jsii would be to enforce that all dependencies are always specified as "peer dependencies". This forbids closures from including more than a single version of a certain module and essentially levels the playing field amongst jsii languages.

See: #361