Validate registrations when app is launched.

[mwhittaker]

This PR introduces a validateRegistrations function that checks the validity of component registrations. For now, it simply checks that every weaver.Ref[T] field refers to a registered component interface T. This mistake is actually quite common, when you add a component and forget to run weaver generate.

Before this PR, if a non-main component had a weaver.Ref[T] to an unregistered component, the app would run but silently not work. With this change, all weavelets will imediately crash, and for go run . and weaver multi deploy, the app will terminate immediately with a helpful error message:

$ weaver multi deploy weaver.toml
start main process: NewEnvelope: connect to weavelet: read protobuf
length: EOF
-----BEGIN STDERR-----
2023/08/04 13:31:50 component implementation struct main.odd has field
weaver.Ref[main.this_is_not_a_registered_component], but component
main.this_is_not_a_registered_component was not registered; maybe you
forgot to run 'weaver generate'

-----END STDERR-----
exit status 1

Alternatives

Srdjan had a great idea of checking the validity of weaver.Ref[T]s at compile time, but I couldn't think of any way to do that. If we do think of something, we should switch to it.

I did have one idea that turned out to be terrible. We could require users to embed a special weaver.Interface interace into their component interfaces:

// In weaver package.
type Interface interface { isComponentInterface() }
type Ref[T Interface] struct { ... }
type Implements[T Interface] struct { ... }
func (Implements[T]) isComponentInterface()

// In user's code.
type ExampleComponent {
    weaver.Interface
    Foo(context.Context) error
}

This is not only ugly and super janky, it also doesn't work. It checks that a weaver.Ref[T] refers to a component interface T, but it doesn't check that T was actually registered.

[spetrovic77]

Very nice!

[spetrovic77]

nit: s/an/a/