cmd/go: document import "C" check from CL 129062

Added this locally but then broke the first rule of Gerrit
and clicked Submit instead of running "git submit".

Change-Id: I83c28d9151c566e9b2092e2613d67731a5d64beb
Reviewed-on: https://go-review.googlesource.com/129678
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>

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

@@ -49,12 +49,19 @@ Files:
 		if err != nil {
 			return nil, nil, fmt.Errorf("reading %s: %v", name, err)
 		}
-		// import "C" is implicit requirement of cgo tag
+
+		// import "C" is implicit requirement of cgo tag.
+		// When listing files on the command line (explicitFiles=true)
+		// we do not apply build tag filtering but we still do apply
+		// cgo filtering, so no explicitFiles check here.
+		// Why? Because we always have, and it's not worth breaking
+		// that behavior now.
 		for _, path := range list {
 			if path == `"C"` && !tags["cgo"] && !tags["*"] {
 				continue Files
 			}
 		}
+
 		if !explicitFiles && !ShouldBuild(data, tags) {
 			continue
 		}