Add tags to the fields of Source that lower-cases their names for JSON.
The implementation still treats Source specially for performance, but
now the result would be identical if it did not.
Change-Id: I5fd2e500f1a301db62af87be8b877ecd954a26ec
Reviewed-on: https://go-review.googlesource.com/c/go/+/494035
Run-TryBot: Jonathan Amsterdam <jba@google.com> Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>