p3: Consider bringing back the imports world for wasi-http

[ricochet]

Consider bringing back the imports world for wasi-http.

A valid usage of wasi-http is to consume only the types (no exports) and use some other export other than the worlds in wasi:http.

While updating the wasi-testsuite with the latest rc, there is a test that uses the HTTP response type even though it's not serving/handling or making client requests. In this scenario this was a test in wasi:cli/run. To workaround the lack of an imports world, I had to implement the handler trait: https://github.com/WebAssembly/wasi-testsuite/pull/200/files#diff-496dfc45abfcebe766d04da19e6c044b10b192b03b80fe7bf97788fe4bc42c14R67-R72

[lukewagner]

Agreed that consuming only the types and client.send function is indeed a valid use case, but would this be equialently achievable by adding an import wasi:http/types or import wasi:http/client (which will implicitly pull in types via use) to whatever world that would've had include wasi:http/imports?