refactor/mvpkg: rewrite external test packages.

Rename the package name of the external test package, e.g. <from>_test to <to>_test.
Rewrite the import statements of external test packages which import the renamed package.

Fixes #10507

Change-Id: Iad702189469c54776e55ed4a821610bd9977618c
Reviewed-on: https://go-review.googlesource.com/12637
Reviewed-by: Alan Donovan <adonovan@google.com>

refactor/rename/mvpkg.go

@@ -241,6 +241,30 @@ func (m *mover) move() error {
 		f.Name.Name = newName // change package decl
 		filesToUpdate[f] = true
 	}
+
+	// Look through the external test packages (m.iprog.Created contains the external test packages).
+	for _, info := range m.iprog.Created {
+		// Change the "package" declaration of the external test package.
+		if info.Pkg.Path() == m.from+"_test" {
+			for _, f := range info.Files {
+				f.Name.Name = newName + "_test" // change package decl
+				filesToUpdate[f] = true
+			}
+		}
+
+		// Mark all the loaded external test packages, which import the "from" package,
+		// as affected packages and update the imports.
+		for _, imp := range info.Pkg.Imports() {
+			if imp.Path() == m.from {
+				m.affectedPackages[info.Pkg.Path()] = true
+				m.iprog.Imported[info.Pkg.Path()] = info
+				if err := importName(m.iprog, info, m.from, path.Base(m.from), newName); err != nil {
+					return err
+				}
+			}
+		}
+	}
+
 	// Update imports of that package to use the new import name.
 	// None of the subpackages will change their name---only the from package
 	// itself will.

refactor/rename/mvpkg_test.go

@@ -203,6 +203,38 @@ var _ a.T
 import "bar/a"
 
 var _ a.T
+`,
+			},
+		},
+
+		// External test packages
+		{
+			ctxt: buildutil.FakeContext(map[string]map[string]string{
+				"foo": {
+					"0.go":      `package foo; type T int`,
+					"0_test.go": `package foo_test; import "foo"; var _ foo.T`,
+				},
+				"baz": {
+					"0_test.go": `package baz_test; import "foo"; var _ foo.T`,
+				},
+			}),
+			from: "foo", to: "bar",
+			want: map[string]string{
+				"/go/src/bar/0.go": `package bar
+
+type T int
+`,
+				"/go/src/bar/0_test.go": `package bar_test
+
+import "bar"
+
+var _ bar.T
+`,
+				"/go/src/baz/0_test.go": `package baz_test
+
+import "bar"
+
+var _ bar.T
 `,
 			},
 		},