Disable submodule exports

[JLHwung]

Q	A
License	MIT

This PR adds exports: "lib/index.js" to every package except @babel/runtime(-corejs[23])?, which shall be addressed in #10853 . By doing so we are stating explicitly that every package does not offer submodule exports, i.e. import "@babel/core/src/config" will bail because this file belongs to internal implementation details.

Even we don't specify exports: false, the following imports will still bail

import "@babel/core/src/config/index.js";

because the subpath ./src/config/index.js is not defined as export path in package.json. So it should not break anyone.

Note that it is a breaking change because exports takes precedence over main so the following will bail on node.js >= 13.1 or when --experimental-modules is activated:

const babelVersion = require("@babel/core/package.json").version

[nicolo-ribaudo]

Does this also applies when using require instead of import?
If it does, then it will break some popular projects:

• vue-cli
• ember-cli-babel

We should probably consider exposing @babel/preset-env/lib/targets-parser as a separate helper package.

[JLHwung]

Does this also applies when using require instead of import?

Yes, because exports take over main, which means Node.js 13 will have different module resolution behaviour than previous versions if package exports presents. Therefore, it is a breaking change.

People may use import "@babel/core/lib/whatever" in the wild and 8.x is expected in weeks. As you said there are some cases when the submodule exports is inevitable, so I propose to limit the scope of this PR:

1. add exports: false to every @babel/* packages with only lib/index.js. This means that
   either require("@babel/foo/lib") or require("@babel/foo/lib/index.js" will throw. The impact is minimal since one can and should always use require("@babel/foo")

2. for other packages, add necessary exports for compatibility with popular downstream tools. We can always create new package in 8.x if it is reasonable. Finally we can expect to remove unintended submodule exports on 9.x

[babel-bot]

Build successful! You can test your changes in the REPL here: https://babeljs.io/repl/build/24590/

[JLHwung]

This PR has been revamped according to the latest best practise: https://nodejs.org/api/esm.html#esm_main_entry_point_export, to specify both exports and main as relative path starting with .

{
  "main": "./lib/index.js",
  "exports": "./lib/index.js"
}

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[JLHwung]

The e2e test is failing because vue-cli invokes require("@babel/core/package.json") and package.json entry is not defined in @babel/core. The node communities are still discussing whether package.json should be allowed: nodejs/node#33460

Before decisions are made by node TSC, I consider package.json as our package metadata and thus it falls into part of our public interfaces. I will add ./package.json entries to every packages.