Fixes #26134. Adds support for webpack 5 (while keeping webpack 4 compat) to dependency-extraction-webpack-plugin. Spinoff from #26382. Description of changes I had to make follows.

Remove type annotations, commit a hand-crafted types.d.ts
After discussion with @sirreal in #26382 (comment), I decided to stop typechecking the plugin source code with TypeScript. Webpack types are not complete enough to make it work, and making a plugin that supports both webpack 4 and 5 is an impossible task. To make types available for consumers of the plugin, I added a hand-crafted types.d.ts file that describes the plugin's public interface, i.e., the constructor and its options. And it works in my Atom IDE:

Stop using the webpack-sources package if possible
Depending on and importing the webpack-sources package is legacy now. A plugin can access the APIs on a webpack.sources property of the webpack package export.

Descend into concatenated modules
The ExternalModule instances can now be concatenated into a common module. We need to descend into ConcatenatedModule.modules array to find all externals.

Don't read compiler options before defaults were applied
In webpack 5, the default options are not applied yet when Plugin.apply is called. Postpone reading the output.filename option to the emit hook where the value is used. See also: https://webpack.js.org/blog/2020-10-10-webpack-5-release/#new-plugin-order

Then there are changes that are in unit tests only:

Change how ExternalModule instances are identified
In webpack 5, ExternalModule instanced no longer have the external boolean flag. We need to identify them by the externalType field presence.

Sort modules by .identifier() rather than implicitly by .toString()
Produces more stable sorting of the modules, preventing test failures due to implementation details changes. In webpack 5, the toString() output includes a debugId value, which is an incremental counter incremented on each Module instantiation. Looking at .identifier() produces more stable sorting key.

Remove the with-externs unit test that depends on webpack internals
There was a with-interns unit test that checked for behavior of two ExternalsPlugin instances, one from config, one from this plugin, that match the same request. But this tests webpack internals rather than anything else.

In webpack 4, the module factories were processed by a SyncWaterfallHook, using the last factory value produced by the hook for a given request. In webpack 5, the hook is an AsyncSeriesBailHook, and it bails on the first plugin returning a valid factory.

So, even if two conflicting ExternalsPlugin instances were declared in the same order, the behavior is different in v4 vs v5.

How to test:
The full webpack 5 upgrade is stacked on top of this PR in #26382, and it builds cleanly including all unit and e2e tests.

This PR alone still uses webpack 4 with the updated dependency-extraction-webpack-plugin. And is also green.