Breaking: use named export in metro-cache-key

Summary:
Switch `metro-cache-key` from a function-typed `module.exports` to a named `getCacheKey` export.

This will help convert the module to ESM syntax with generated TS types without any issues with `default` export interop. It's also more IDE-friendly.

Changelog:
```
 - **[Breaking]**: metro-cache-key: Use a named `getCacheKey` export in place of default function export
```

Reviewed By: vzaidman

Differential Revision: D78157825

fbshipit-source-id: 30f12fc905d4cb46dce20898ef8246377b30661b

Author: robhogan

packages/metro-cache-key/src/__tests__/index-test.js

@@ -13,7 +13,7 @@
 
 jest.mock('fs', () => new (require('metro-memory-fs'))());
 
-const getCacheKey = require('../index');
+const {getCacheKey} = require('../index');
 const fs = require('fs');
 
 beforeAll(() => {

packages/metro-cache-key/src/index.js

@@ -23,4 +23,4 @@ function getCacheKey(files: Array<string>): string {
     .digest('hex');
 }
 
-module.exports = getCacheKey;
+module.exports = {getCacheKey};

packages/metro-transform-worker/src/index.js

@@ -41,7 +41,7 @@ const generate = require('@babel/generator').default;
 const babylon = require('@babel/parser');
 const types = require('@babel/types');
 const {stableHash} = require('metro-cache');
-const getCacheKey = require('metro-cache-key');
+const {getCacheKey} = require('metro-cache-key');
 const {
   fromRawMappings,
   functionMapBabelPlugin,

packages/metro/src/DeltaBundler/getTransformCacheKey.js

@@ -16,7 +16,7 @@ import type {JsTransformerConfig} from 'metro-transform-worker';
 
 const VERSION = require('../../package.json').version;
 const crypto = require('crypto');
-const getCacheKey = require('metro-cache-key');
+const {getCacheKey} = require('metro-cache-key');
 
 type CacheKeyProvider = {
   getCacheKey?: JsTransformerConfig => string,