Change wasi:clocks from include to import in wasi:http/proxy

[yoshuawuyts]

This changes the wasi:clocks include statement to individual import statements in wasi:http/proxy. This came out of a conversation with @sunfishcode yesterday, who pointed out that if we're going to add timezones to wasi:clocks (WebAssembly/wasi-clocks#61), wasi:http/proxy may not actually want to provide timezone support.

By switching from a catch-all include to individual import statements, we can decide later whether this is something we'd like to include. But conservatively we can start with the assumption that we won't.

This has some precedent in how we're handling our re-exports of wasi:cli: wasi:http/proxy includes wasi:cli/stdin and wasi:cli/stdout - but it does for example not include support for files or sockets.

[lukewagner]

Makes sense to me