wac plug with registry support

[calvinrp]

Updates Warg to 0.6.0 release with support for federating across multiple registries.

Adds wac plug registry support (feature flag still but a default one).

Existing behavior is unchanged:
wac plug my-component.wasm --plug helpful.wasm -o plugged.wasm

But now, you can mix in registry package names or local file paths. In any combination. Kind of fun.

It determines whether to look in the registry or local path by trying to parse as valid namespace:package-name. Also, parses with versions in namespace:package-name@0.1.0 syntax. If it is a valid package name, then it looks in the registry otherwise tries local file paths. So path/some-file.wasm would be unambiguously a local file path.

wac plug macovedj:hashimap-service --plug macovedj:my-component@0.0.1 -o plugged.wasm
(those are packages from @macovedj WasmIO demo and that command will work if Warg is configured)

or

wac plug hashimap-service.wasm --plug macovedj:my-component@0.0.1 -o plugged.wasm

[calvinrp]

@macovedj thanks for the review, but I definitely want the other maintainers to weigh in on this.

[fibonacci1729]

This looks good to me; I'd like to give @rylev a chance to review when he returns next week.

[rylev]

Looks good to me. I just would like to see the comment about the usage of println! in the plug command addressed.