cmd/go: ignore import "C" files in module loader in non-cgo mode

Obviously, including files that import "C" when cgo is disabled is wrong.
The package load step correctly excludes them and finds no files at all,
which then causes a failure.

Fixes #26927.

Change-Id: I00e6d6450e783d467d20bde99e91240ecb0db837
Reviewed-on: https://go-review.googlesource.com/129062
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
Reviewed-by: David du Colombier <0intro@gmail.com>

src/cmd/go/internal/imports/scan.go

@@ -37,6 +37,7 @@ func scanFiles(files []string, tags map[string]bool, explicitFiles bool) ([]stri
 	imports := make(map[string]bool)
 	testImports := make(map[string]bool)
 	numFiles := 0
+Files:
 	for _, name := range files {
 		r, err := os.Open(name)
 		if err != nil {
@@ -48,6 +49,12 @@ func scanFiles(files []string, tags map[string]bool, explicitFiles bool) ([]stri
 		if err != nil {
 			return nil, nil, fmt.Errorf("reading %s: %v", name, err)
 		}
+		// import "C" is implicit requirement of cgo tag
+		for _, path := range list {
+			if path == `"C"` && !tags["cgo"] && !tags["*"] {
+				continue Files
+			}
+		}
 		if !explicitFiles && !ShouldBuild(data, tags) {
 			continue
 		}

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

@@ -43,6 +43,10 @@ stdout example.com/m/useunsafe
 [cgo] stdout example.com/m/useC
 [!cgo] ! stdout example.com/m/useC
 
+env CGO_ENABLED=0
+go list -f '{{.ImportPath}}: {{.Match}}' all ... example.com/m/... ./... ./xyz...
+! stdout example.com/m/useC
+
 -- m/go.mod --
 module example.com/m