Only use .flow.js extensions when there is an adjacent plain JS that registers Babel (#1546)

Summary:
Currently, we have a smattering of `*.flow.js` files in Metro. These mean either of two things:
 1) Modules that only export Flow types
 2) Modules that are written in Flow, to distinguish them from a colocated plain JS file.

Type 1) doesn't really have any significance and isn't enforced - we still build and publish (usually empty) `foo.flow.js` files and even accompanying `foo.flow.js.flow` files, and there's nothing to stop them containing runtime code (indeed, with enums, they do).

This diff:
 - Renames all files of type 1) to drop the `.flow`.
 - Adds a test to ensure all `.flow.js` files have an adjacent `.js` file with the expected content.

This is a prerequisite for auto-generating TS types for Metro - by establishing and enforcing a convention that `foo.js` and `foo.flow.js` are a Babel-registering entry point and an implementation file respectively, we can teach the API generator that `foo.flow.js` should generate `foo.d.ts`, and `foo.js` should be ignored.

Changelog: Internal

Pull Request resolved: #1546

Test Plan:
- Flow/CI
 - Test verifies when JS has unexpected content:

```
$ echo "module.exports = 'illegal';" >> xplat/js/tools/metro/packages/metro/src/index.js
$ js1 jest subpackages
Determining test suites to run...
 FAIL  xplat/js/tools/metro/scripts/__tests__/subpackages-test.js
<...>

  ● packages/metro › all .flow.js files have an adjacent babel-registering entry point

    expect(received).toEqual(expected) // deep equality

    - Expected  - 1
    + Received  + 2

    @@ -7,7 +7,8 @@

      try {
        require('metro-babel-register').unstable_registerForMetroMonorepo();
      } catch {}

    - module.exports = require('./index.flow');
    + module.exports = require('./index.flow');
    + module.exports = 'illegal';
      ↵

      152 |       const endOfHeader = content.indexOf('*/\n') + 3;
      153 |       expect(endOfHeader).toBeGreaterThan(3);
    > 154 |       expect(content.slice(endOfHeader)).toEqual(`
          |                                          ^
      155 | 'use strict';
      156 |
      157 | /*::

      at Object.toEqual (scripts/__tests__/subpackages-test.js:154:42)

Test Suites: 1 failed, 1 total
Tests:       1 failed, 180 passed, 181 total
Snapshots:   0 total
Time:        0.599 s, estimated 1 s
```

Reviewed By: huntie

Differential Revision: D79566838

Pulled By: robhogan

fbshipit-source-id: 68c2a17b50c8fe579e856dbca24ea60f7c8bd8a7

Author: robhogan

docs/Configuration.md

@@ -20,7 +20,7 @@ const HttpStore = require('./stores/HttpStore');
 
 export type {Options as FileOptions} from './stores/FileStore';
 export type {Options as HttpOptions} from './stores/HttpStore';
-export type {CacheStore} from './types.flow';
+export type {CacheStore} from './types';
 
 module.exports.AutoCleanFileStore = AutoCleanFileStore;
 module.exports.Cache = Cache;

packages/metro-cache/src/index.js

@@ -11,7 +11,7 @@
 
 'use strict';
 
-import type {PerfLogger, RootPerfLogger} from '../configTypes.flow';
+import type {PerfLogger, RootPerfLogger} from '../types';
 
 const defaultCreateModuleIdFactory = require('metro/private/lib/createModuleIdFactory');
 

packages/metro-cache/src/types.flow.js packages/metro-cache/src/types.jspackages/metro-cache/src/types.flow.js renamed to packages/metro-cache/src/types.js

@@ -11,7 +11,7 @@
 
 'use strict';
 
-import type {ConfigT} from '../configTypes.flow';
+import type {ConfigT} from '../types';
 
 const {
   DEFAULT_METRO_MINIFIER_PATH,

packages/metro-config/src/defaults/defaults.js

@@ -12,7 +12,7 @@
 'use strict';
 
 /*::
-export type * from './configTypes.flow';
+export type * from './types';
 */
 
 try {

packages/metro-config/src/defaults/index.js

@@ -11,7 +11,7 @@
 
 'use strict';
 
-import type {ConfigT, InputConfigT, YargArguments} from './configTypes.flow';
+import type {ConfigT, InputConfigT, YargArguments} from './types';
 
 const getDefaultConfig = require('./defaults');
 const validConfig = require('./defaults/validConfig');

packages/metro-config/src/index.js

@@ -23,7 +23,7 @@ import type {
   Module,
   ReadOnlyGraph,
   SerializerOptions,
-} from 'metro/private/DeltaBundler/types.flow';
+} from 'metro/private/DeltaBundler/types';
 import type {Reporter} from 'metro/private/lib/reporting';
 import type MetroServer from 'metro/private/Server';
 

packages/metro-config/src/loadConfig.js

@@ -11,5 +11,5 @@
 import getDefaultConfig from './defaults';
 import {loadConfig, mergeConfig, resolveConfig} from './loadConfig';
 
-export * from './configTypes';
+export * from './types';
 export {loadConfig, mergeConfig, resolveConfig, getDefaultConfig};

…ges/metro-config/src/configTypes.flow.js packages/metro-config/src/types.jspackages/metro-config/src/configTypes.flow.js renamed to packages/metro-config/src/types.js packages/metro-config/src/configTypes.flow.js renamed to packages/metro-config/src/types.js

@@ -8,7 +8,7 @@
  * @oncall react_native
  */
 
-import type {ConfigT, InputConfigT, YargArguments} from './configTypes';
+import type {ConfigT, InputConfigT, YargArguments} from './types';
 
 export interface CosmiConfigResult {
   filepath: string;