cmd/go: don't compute Embed fields if they're not needed
If the user provides the -json flag to explicitly specify fields, but doesn't specify any *Embed* field, skip computing the embed fields.
This enhances the initial implementation of #29666.
Change-Id: I60e86fb25a445689aecbcc7f3f3f88e0f37a0fc5
GitHub-Last-Rev:
2795c195bf995f798a45e928becebc253c89b9d6
GitHub-Pull-Request: golang/go#58522
Reviewed-on: https://go-review.googlesource.com/c/go/+/468075
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>