cmd/go: print a "found" line for each package found instead of each module added
cmd/go: print a "found" line for each package found instead of each module added

We currently print a "go: finding" line for each missing package
during import resolution. However, we are only printing a "go: found"
line for each module: if a given module provides multiple packages, we
don't indicate the module that we found for the second and later
packages.

Before this change:

	$ GO111MODULE=on go get golang.org/x/tools/cmd/html2article@78f9822548c13e2c41cc8039d1492a111240db07
	go: found golang.org/x/tools/cmd/html2article in golang.org/x/tools v0.0.0-20190214195451-78f9822548c1
	go: finding module for package golang.org/x/net/html
	go: finding module for package golang.org/x/net/html/atom
	go: downloading golang.org/x/net v0.0.0-20200202094626-16171245cfb2
	go: found golang.org/x/net/html in golang.org/x/net v0.0.0-20200202094626-16171245cfb2

After:

	$ GO111MODULE=on go get golang.org/x/tools/cmd/html2article@78f9822548c13e2c41cc8039d1492a111240db07
	go: found golang.org/x/tools/cmd/html2article in golang.org/x/tools v0.0.0-20190214195451-78f9822548c1
	go: finding module for package golang.org/x/net/html/atom
	go: finding module for package golang.org/x/net/html
	go: found golang.org/x/net/html in golang.org/x/net v0.0.0-20200202094626-16171245cfb2
	go: found golang.org/x/net/html/atom in golang.org/x/net v0.0.0-20200202094626-16171245cfb2

Updates #26152
Updates #33284

Change-Id: I221548749e36bfd6a79efe5edc3645dc5319fd6f
Reviewed-on: https://go-review.googlesource.com/c/go/+/219437
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

akutz/go,cloudflare/go,CAFxX/go,cloudflare/go,CAFxX/go,golang/go,akutz/go,akutz/go,cloudflare/go,nawawi/go,cloudflare/go,golang/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,cloudflare/go,CAFxX/go,nawawi/go,cloudflare/go,CAFxX/go,golang/go,CAFxX/go,golang/go,nawawi/go,nawawi/go,golang/go,nawawi/go,CAFxX/go,cloudflare/go,nawawi/go,akutz/go,golang/go,golang/go,akutz/go,akutz/go,golang/go,CAFxX/go,nawawi/go