cmd/go: only try to clean executables for package main

Fixes #5665.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/12807044

src/cmd/go/clean.go

@@ -137,22 +137,38 @@ func clean(p *Package) {
 	}
 
 	_, elem := filepath.Split(p.Dir)
-	allRemove := []string{
+	var allRemove []string
+
+	// Remove dir-named executable only if this is package main.
+	if p.Name == "main" {
+		allRemove = append(allRemove,
 			elem,
 			elem+".exe",
+		)
+	}
+
+	// Remove package test executables.
+	allRemove = append(allRemove,
 		elem+".test",
 		elem+".test.exe",
-	}
+	)
+
+	// Remove a potental executable for each .go file in the directory that
+	// is not part of the directory's package.
 	for _, dir := range dirs {
 		name := dir.Name()
 		if packageFile[name] {
 			continue
 		}
 		if !dir.IsDir() && strings.HasSuffix(name, ".go") {
+			// TODO(adg,rsc): check that this .go file is actually
+			// in "package main", and therefore capable of building
+			// to an executable file.
 			base := name[:len(name)-len(".go")]
 			allRemove = append(allRemove, base, base+".exe")
 		}
 	}
+
 	if cleanN || cleanX {
 		b.showcmd(p.Dir, "rm -f %s", strings.Join(allRemove, " "))
 	}