[ServerRenderer] Move fizz external runtime implementation to react-d…

…om-bindings (facebook#25617)

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Following
[comment](facebook#25437 (comment))
in facebook#25437 , the external runtime implementation should be moved from
`react-dom` to `react-dom-bindings`.

I did have a question here:
I set the entrypoint to `react-dom/unstable_server-external-runtime.js`,
since a.) I was following facebook#25436 as an example and b.)
`react-dom-bindings` was missing a `README.md` and `npm/`. This also
involved adding the external runtime to `package.json`.
However, the external runtime isn't really a `react-dom` entrypoint. Is
this change alright, or should I change the bundling code instead?

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packages/react-dom/src/server/ReactDOMServerExternalRuntime.js → packages/react-dom-bindings/src/server/ReactDOMServerExternalRuntime.js

@@ -11,7 +11,7 @@ import {
   completeBoundaryWithStyles,
   completeBoundary,
   completeSegment,
-} from '../../../react-dom-bindings/src/server/fizz-instruction-set/ReactDOMFizzInstructionSet';
+} from './fizz-instruction-set/ReactDOMFizzInstructionSet';
 
 // Intentionally does nothing. Implementation will be added in future PR.
 // eslint-disable-next-line no-unused-vars

packages/react-dom/package.json

@@ -65,6 +65,7 @@
     "./profiling": "./profiling.js",
     "./test-utils": "./test-utils.js",
     "./unstable_testing": "./unstable_testing.js",
+    "./unstable_server-external-runtime": "./unstable_server-external-runtime.js",
     "./src/*": "./src/*",
     "./package.json": "./package.json"
   },

packages/react-dom/unstable_server-external-runtime.js

@@ -7,4 +7,4 @@
  * @flow
  */
 
-export * from 'react-dom/src/server/ReactDOMServerExternalRuntime';
+export * from 'react-dom-bindings/src/server/ReactDOMServerExternalRuntime';

scripts/rollup/bundles.js

@@ -357,7 +357,7 @@ const bundles = [
   {
     bundleTypes: [BROWSER_SCRIPT],
     moduleType: RENDERER,
-    entry: 'react-dom/src/server/ReactDOMServerExternalRuntime.js',
+    entry: 'react-dom/unstable_server-external-runtime',
     outputPath: 'unstable_server-external-runtime.js',
     global: 'ReactDOMServerExternalRuntime',
     minifyWithProdErrorCodes: false,

scripts/shared/inlinedHostConfigs.js

@@ -15,7 +15,7 @@ module.exports = [
       'react-dom/src/server/ReactDOMFizzServerNode.js',
       'react-dom/static.node',
       'react-dom/server-rendering-stub',
-      'react-dom/src/server/ReactDOMServerExternalRuntime.js',
+      'react-dom/unstable_server-external-runtime',
       'react-server-dom-webpack/writer.node.server',
       'react-server-dom-webpack',
     ],
@@ -52,7 +52,7 @@ module.exports = [
       'react-dom/src/server/ReactDOMFizzServerBrowser.js',
       'react-dom/static.browser',
       'react-dom/server-rendering-stub',
-      'react-dom/src/server/ReactDOMServerExternalRuntime.js',
+      'react-dom/unstable_server-external-runtime',
       'react-server-dom-webpack/writer.browser.server',
       'react-server-dom-webpack',
     ],