Short circuit for buildmod vendor

Author: rohnnyjoy

src/cmd/go/internal/modload/import.go

@@ -546,10 +546,12 @@ func queryImport(ctx context.Context, path string, rs *Requirements) (module.Ver
 		return module.Version{}, &ImportMissingError{Path: path, isStd: true}
 	}
 
-	if cfg.BuildMod == "readonly" && !allowMissingModuleImports {
+	if (cfg.BuildMod == "readonly" || cfg.BuildMod == "vendor") && !allowMissingModuleImports {
 		// In readonly mode, we can't write go.mod, so we shouldn't try to look up
 		// the module. If readonly mode was enabled explicitly, include that in
 		// the error message.
+		// In vendor mode, we cannot use the network or module cache, so we
+		// shouldn't try to look up the module
 		var queryErr error
 		if cfg.BuildModExplicit {
 			queryErr = fmt.Errorf("import lookup disabled by -mod=%s", cfg.BuildMod)

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

@@ -27,8 +27,7 @@ cmp go.mod go.mod.orig
 
 ! go list -mod=vendor all
 ! stderr '^go: inconsistent vendoring'
-stderr 'go: finding module for package example.com/badedit'
-stderr 'cannot query module due to -mod=vendor'
+stderr 'cannot find module providing package example.com/badedit: import lookup disabled by -mod=vendor'
 
 # When we set -mod=mod, the go version should be updated immediately,
 # to the current version, converting the requirements from eager to lazy.

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

@@ -22,8 +22,7 @@ cd broken
 ! go build -mod=readonly
 stderr 'disabled by -mod=readonly'
 ! go build -mod=vendor
-stderr 'go: finding module for package golang.org/x/net/http2/hpack'
-stderr 'http.go:5:2: cannot query module due to -mod=vendor'
+stderr 'http.go:5:2: cannot find module providing package golang.org/x/net/http2/hpack: import lookup disabled by -mod=vendor'
 
 
 # ...even if they explicitly use the "cmd/vendor/" or "vendor/" prefix.