Allow path expansion/modification for the `deps` directory in `generate!`

[mkatychev]

Feature

allow wit_bindgen::generate! (and wasmtime::bindgen!) to link a dependency tree that is not a sibling path to the root WIT package to be generated:

bindgen!({
    world: "foo",
    path: "other/path/to/wit",
    deps: "path/to/foo/dir/deps", // behaves as "other/path/to/wit/dir"
    // ...
});

Benefit

This would allow aggregation of dependency trees for multiple packages that share different directories

Implementation

Likely u directory association inside of bindgen! wasmtime::Config

Alternatives

Symlinking, this is not supported on windows 😢

[nonik0]

Also additionally allowing the provided path and/or potentially deps paths to expand to allow the use of env vars to encapsulate the "path expansion" part of this issue. This would make referencing shared deps between components more robust through the use of env vars in the provided paths, rather than relying on relative paths that could break or absolute paths that are not portable.