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

#### 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:

```rs
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 😢