go: fix Go self-call schema names and cover dependency use

[grouville]

This PR is a follow-up against eunomie's workspace-go-no-codegen-at-runtime branch.

That branch changes how Go modules with SELF_CALLS provide the schema used to generate their own Go client. My manual review led to those findings:

• Go self-calls worked when called directly.
• Go self-calls also worked when the SELF_CALLS module was used as a dependency.
• They also worked through a transitive dependency.
• A separate real issue showed up with exported fields.

The actual bug

A Go module can expose fields on its root object:

type Test struct {
  Message string
}

The engine exposes that field as message, but the new self-call schema emitter was exposing it as Message.

That mismatch can break the generated Go client. In the failing case, generated code could end up with a field and a method using the same Go name, so the code would not compile.

What changed:

• Emit exported fields in the self-call schema with the same name the engine uses --> Message now becomes message.
• Extend the existing Go self-call fixture with a Message field.
• Add an integration test that queries message, proving the generated client compiles and the schema name is correct.
• Add dependency fixtures to lock down the review concern even though the exact reported failure did not reproduce.

Dependency coverage added:

• caller -> dep
• caller -> middle -> dep

The dep module is the only module with SELF_CALLS enabled. That keeps the tests focused on the case from the review.

The new tests verify:

• a dependency function can self-call through dag.Dep()
• a container returned by a dependency self-call is still usable by the caller
• self-calls also work from a secondary object
• self-calls work when the SELF_CALLS module is a transitive dependency

Tested with:

dagger call engine-dev test --pkg ./core/integration --run '^TestModule/TestSelfCalls' --test-verbose --count=1 --failfast --timeout=20m

Result: 17 passed.