Update naming convention in jest-watcher (#7315)

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<!-- Please remember to update CHANGELOG.md in the root of the project if you have not done so. -->

## Summary

This is part of #4969.
It standardizes file names in packages/jest-watcher under Facebook naming conventions.

## Test plan

Running `yarn run clean-all` followed by `yarn` and `yarn test` is not completely clean on my machine, but it yields the same results in this branch and `master`

CHANGELOG.md

@@ -65,6 +65,7 @@
 
 ### Chore & Maintenance
 
+- `[jest-watcher]` Standardize filenames ([#7314](https://github.com/facebook/jest/pull/7314))
 - `[jest-circus]` Standardize file naming in `jest-circus` ([#7301](https://github.com/facebook/jest/pull/7301))
 - `[docs]` Add synchronous test.each setup ([#7150](https://github.com/facebook/jest/pull/7150))
 - `[docs]` Add `this.extend` to the Custom Matchers API reference ([#7130](https://github.com/facebook/jest/pull/7130))

packages/jest-watcher/src/index.js

@@ -7,9 +7,9 @@
  * @flow
  */
 
-export {default as BaseWatchPlugin} from './base_watch_plugin';
-export {default as JestHook} from './jest_hooks';
-export {default as PatternPrompt} from './pattern_prompt';
+export {default as BaseWatchPlugin} from './BaseWatchPlugin';
+export {default as JestHook} from './JestHooks';
+export {default as PatternPrompt} from './PatternPrompt';
 export * from './constants';
 export {default as Prompt} from './lib/Prompt';
-export * from './lib/pattern_mode_helpers';
+export * from './lib/patternModeHelpers';

packages/jest-watcher/src/lib/__tests__/format_test_name_by_pattern.test.js → packages/jest-watcher/src/lib/__tests__/formatTestNameByPattern.test.js

@@ -9,7 +9,7 @@
 
 'use strict';
 
-import formatTestNameByPattern from '../format_test_name_by_pattern';
+import formatTestNameByPattern from '../formatTestNameByPattern';
 
 describe('for multiline test name returns', () => {
   const testNames = [

packages/jest-watcher/src/lib/__tests__/scroll_list.test.js → packages/jest-watcher/src/lib/__tests__/scroll.test.js

@@ -1,6 +1,6 @@
 // Copyright (c) 2014-present, Facebook, Inc. All rights reserved.
 
-import scroll from '../scroll_list';
+import scroll from '../scroll';
 
 it('When offset is -1', () => {
   expect(scroll(25, {max: 10, offset: -1})).toEqual({