cmd/go/internal/modload: do not fetch modules in searchPackages if -mod=vendor is set

Updates #30228
Updates #30241

Change-Id: I6b5f842d00ba273ed241abe55a1ea71c105ec284
Reviewed-on: https://go-review.googlesource.com/c/go/+/165377
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>

src/cmd/go/internal/modload/search.go

@@ -112,6 +112,11 @@ func matchPackages(pattern string, tags map[string]bool, useStd bool, modules []
 		walkPkgs(cfg.GOROOTsrc, "")
 	}
 
+	if cfg.BuildMod == "vendor" {
+		walkPkgs(filepath.Join(ModRoot(), "vendor"), "")
+		return pkgs
+	}
+
 	for _, mod := range modules {
 		if !treeCanMatch(mod.Path) {
 			continue

src/cmd/go/testdata/script/mod_vendor_build.txt

@@ -19,6 +19,12 @@ cmp go.mod go.mod.good
 go list -mod=vendor
 cmp go.mod go.mod.good
 
+# With a clean (and empty) module cache, 'go list -mod=vendor' should not download modules.
+go clean -modcache
+env GOPROXY=off
+! go list ...
+go list -mod=vendor ...
+
 -- go.mod --
 module m