Chore: Add test to ensure manifest-less local packages are installable (

yarnpkg#4456)

**Summary**

Refs yarnpkg#4257. Adds the missing regression test.

**Test plan**

The new test should pass.

__tests__/commands/install/integration.js

@@ -486,13 +486,20 @@ test.concurrent("don't install with file: protocol as default if target is a fil
   return expect(runInstall({lockfile: false}, 'install-file-as-default-no-file')).rejects.toBeDefined();
 });
 
-test.concurrent("don't install with file: protocol as default if target does not have package.json", (): Promise<
+test.concurrent("don't install with implicit file: protocol if target does not have package.json", (): Promise<
   void,
 > => {
   // $FlowFixMe
   return expect(runInstall({lockfile: false}, 'install-file-as-default-no-package')).rejects.toBeDefined();
 });
 
+test.concurrent('install with explicit file: protocol if target does not have package.json', (): Promise<void> => {
+  return runInstall({}, 'install-file-no-package', async config => {
+    expect(await fs.exists(path.join(config.cwd, 'node_modules', 'foo', 'bar.js'))).toEqual(true);
+    expect(await fs.exists(path.join(config.cwd, 'node_modules', 'bar', 'bar.js'))).toEqual(true);
+  });
+});
+
 test.concurrent("don't install with file: protocol as default if target is valid semver", (): Promise<void> => {
   return runInstall({}, 'install-file-as-default-no-semver', async config => {
     expect(await fs.readFile(path.join(config.cwd, 'node_modules', 'foo', 'package.json'))).toMatchSnapshot(

__tests__/fixtures/install/install-file-no-package/package.json

@@ -0,0 +1,6 @@
+{
+  "dependencies": {
+    "foo": "file:bar",
+    "bar": "./bar"
+  }
+}