cmd/go: allow generate to process invalid packages
Allow go generate to process packages that contain invalid code. Ignore
errors when loading the package, but process only files which have a
valid package clause. Set $GOPACKAGE individually for each file, based
on the package clause.
Add test script for go generate and invalid packages.
Fixes #36422
Change-Id: I91ea088346a1548ccd6678b4595a527b948331ff
Reviewed-on: https://go-review.googlesource.com/c/go/+/229097 Reviewed-by: Rob Pike <r@golang.org> Reviewed-by: Jay Conrod <jayconrod@google.com>
Run-TryBot: Jay Conrod <jayconrod@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>