fix: Normalize default_locale before use (#1637)

Normalize `default_locale` by replacing dashes with underscores, like:
https://searchfox.org/mozilla-central/rev/928742d3ea30e0eb4a8622d260041564d81a8468/toolkit/components/extensions/Extension.jsm#999-1011

src/cmd/build.js

@@ -131,10 +131,12 @@ export async function defaultPackageCreator(
 
   let extensionName: string = manifestData.name;
 
-  if (manifestData.default_locale) {
+  let {default_locale} = manifestData;
+  if (default_locale) {
+    default_locale = default_locale.replace(/-/g, '_');
     const messageFile = path.join(
       sourceDir, '_locales',
-      manifestData.default_locale, 'messages.json'
+      default_locale, 'messages.json'
     );
     log.debug('Manifest declared default_locale, localizing extension name');
     extensionName = await getDefaultLocalizedName({

tests/fixtures/dashed-locale/_locales/en_US/messages.json

@@ -0,0 +1,5 @@
+{
+  "extensionName": {
+    "message": "extension with dashed locale"
+  }
+}

tests/fixtures/dashed-locale/manifest.json

@@ -0,0 +1,6 @@
+{
+  "manifest_version": 2,
+  "name": "__MSG_extensionName__",
+  "version": "1.0",
+  "default_locale": "en-US"
+}

tests/unit/test-cmd/test.build.js

@@ -82,7 +82,20 @@ describe('build', () => {
           .then((buildResult) => {
             assert.match(buildResult.extensionPath,
                          /name_of_the_extension-1\.0\.zip$/);
-            return buildResult.extensionPath;
+          })
+    );
+  });
+
+  it('accept a dash in the default_locale field', () => {
+    return withTempDir(
+      (tmpDir) =>
+        build({
+          sourceDir: fixturePath('dashed-locale'),
+          artifactsDir: tmpDir.path(),
+        })
+          .then((buildResult) => {
+            assert.match(buildResult.extensionPath,
+                         /extension_with_dashed_locale-1\.0\.zip$/);
           })
     );
   });