Import name projections don't support exports by interface name

Currently the projection rule for import names looks like:

projection    ::= '/' <label>
label         ::= <word>
                | <label> '-' <word>

This prevents projections for exports using the interfacename format, as these cannot be expressed simply as a label.

Say component foo:bar exports an instance with name bar:baz/qux (i.e. in interfacename format) and we wanted to import this instance from another component, but reference a possible implementation using an unlocked-dep import rather an import of an interface name. We're unable to express this because bar:baz/qux cannot be represented as a projection.

One possible solution:

Extend the projection rule as follows:

projection          ::= '/' <projection-segment>
projection-segment  ::= <label> | '<' <interfacename> '>' 

Or using some other delimiter to denote that the segment is an interface name and not a label.

Start with the import-and-export definitions section of design/mvp/Explainer.md and review the existing projection grammar. Check the issue discussion before proposing syntax for interface-name exports; done means the grammar and accompanying explanation unambiguously support these projections.