relax validity tests to break on slash

Change-Id: I1578a0aa77ee1fc957bb9a93690ed92633d3a65a

vain.go

@@ -50,10 +50,24 @@ func (p Package) String() string {
 	)
 }
 
+func splitPathHasPrefix(path, prefix []string) bool {
+	if len(path) < len(prefix) {
+		return false
+	}
+	for i, p := range prefix {
+		if path[i] != p {
+			return false
+		}
+	}
+	return true
+}
+
 // Valid checks that p will not confuse the go tool if added to packages.
 func Valid(p string, packages []Package) bool {
+	ps := strings.Split(p, "/")
 	for _, pkg := range packages {
-		if strings.HasPrefix(pkg.Path, p) || strings.HasPrefix(p, pkg.Path) {
+		pre := strings.Split(pkg.Path, "/")
+		if splitPathHasPrefix(ps, pre) || splitPathHasPrefix(pre, ps) {
 			return false
 		}
 	}

vain_test.go

@@ -113,6 +113,13 @@ func TestValid(t *testing.T) {
 			in:   "foo/bar/baz",
 			want: true,
 		},
+		{
+			pkgs: []Package{
+				{Path: "a/b"},
+			},
+			in:   "a/bb",
+			want: true,
+		},
 	}
 	for _, test := range tests {
 		got := Valid(test.in, test.pkgs)