schema: Importing other files

[tv42]

Some sort of url-inspired syntax, example.com/myproject/myprotocol.Person v1.

Do we import once with the full url, and then refer with just the basename (or given alias)? import example.com/myproject/myprotocol and then myprotocol.Person v1.

How are the actual *.chi looked up? The Go code gen should use $GOPATH, but also support schemas defined in non-Go projects; and non-Go libraries need a standard lookup mechanism.

Cyclic imports are an error.