add support for cross-crate schema imports

[dwrensha]

See https://groups.google.com/g/sandstorm-dev/c/hdTrPu6gndY/m/heRh-ns1AgAJ

Currently, if someone wants to define schema files that depend on the sandstorm schema files, then they cannot take advantage of the sandstorm-rust crate. To make that work, we need:

1. a way for the downstream crate to find the .capnp files of the upstream crate. Here's an cargo issue I opened about that: DEP_FOO_KEY-like system that can work without "links" rust-lang/cargo#3544
2. a way for capnpc-rust to know the crate name to associate with a given schema. One way to do that might be to add new annotation in rust.capnp: https://github.com/capnproto/capnproto-rust/blob/0f7abd17ba653ea2130185ca20ebb953dc729c5e/capnpc/rust.capnp

[zenhack]

Re: (1), it looks like there's a consensus on the solution and this is just waiting on someone to do the work.

Re: (2), the go implementation does something similar, and it's worked out adequately.

[griff]

I have gotten around the problem by having a checked out version of sandstorm-rust for calling capnpc so the schema files could be found.

And the crate problem can be solved with use. If my crate uses util.capnp it just needs use sandstorm::util_capnp; at the top level to work.

I actually did some work on DEP_FOO_KEY in cargo and got something hacked together but when the cargo tests failed for some reason I but it down and haven't looked at it since.