Support nested vendored dependencies in `bindgen!`

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#### Feature
It seems that the only way to import other packages right now is to vendor them, so I tried to do that by symlinking the dependee
into the dependant `deps` folder (showing dirs only for conciseness):
```
❯ tree -d -l wit
wit
└── deps -> ../../wasi-http/wit
└── deps
├── cli
├── clocks
├── filesystem
├── io
├── random
└── sockets
```

Then i attempt to generate bindings like so:
```rust
bindgen!({
path: "../adhoc/wit",
with: {
"wasi:http/proxy@0.2.1": http_proxy,
}
});
```

and I get the following compile error:
```
error: failed to resolve directory while parsing WIT for path [/Users/mpostma/Documents/code/rust/wasm-experiments/adhoc/wit]

Caused by:
0: failed to parse dependency directory: /Users/mpostma/Documents/code/rust/wasm-experiments/adhoc/wit/deps
1: failed to parse package: /Users/mpostma/Documents/code/rust/wasm-experiments/adhoc/wit/deps/deps
2: no `package` header was found in any WIT file for this package
```
It seems to be because the second level of `deps` doesn't contain any `WIT` file, but only directories.

#### Benefit

I'm not sure what's the current plan, or the recommended way to deal with imports right now. I suspect that all of this will converge to a more streamlined experience, but until then, it would be nice if we could just symlink components as dependency for other components.