This change adds support for testing go.mod files within packagestest. Primarily, if there are markers in the go.mod file, this will copy the contents to a temporary file, build the modcache, then set the contents back.
Updates golang/go#36091
Change-Id: Icb707906eb7fc9e4a06fe043f94f34d9223d84c9
Reviewed-on: https://go-review.googlesource.com/c/tools/+/216839
Run-TryBot: Rohan Challa <rohan@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
