Huge untreeshakable size increase due to `undici`

Describe the bug

@actions/http-client and all the packages like @actions/core are now shipping huge amounts of more JavaScript due to the inclusion of undici in a way that is not tree-shakable. Importing any simple function from @actions/core results in the bloat.

This has happened in the recent versions only. It seems the way undici is used results

To Reproduce
Bundle @actions/http-client using a bundler like Parcel

Expected behavior
Make the dependencies tree-shakable and lazy lodable via dynamic imports

Screenshots

Here's the bundling result in setup-cpp:

After updating the version (625 KB minified). Notice all the undici addition:

dist/legacy/setup-cpp.js                                                                        625.86 KB    1.23s
├── Code from unknown sourcefiles                                                                84.02 KB      0ms
├── node_modules/undici/lib/llhttp/llhttp-wasm.js                                                72.23 KB    119ms
├── node_modules/undici/lib/llhttp/llhttp_simd-wasm.js                                           72.21 KB    119ms
├── node_modules/undici/lib/client.js                                                            25.49 KB    141ms
├── node_modules/@actions/tool-cache/node_modules/semver/semver.js                               18.09 KB    149ms
├── node_modules/undici/lib/fetch/index.js                                                       16.29 KB    140ms
├── node_modules/undici/lib/fetch/request.js                                                      9.18 KB    127ms
├── node_modules/@actions/http-client/src/index.ts                                                9.09 KB      0ms
├── node_modules/undici/lib/fetch/util.js                                                         7.78 KB    114ms
└── node_modules/undici/lib/cache/cache.js                                                         7.2 KB    105ms
└── + 355 more assets

Before updating (237 KB minified)

dist/legacy/setup-cpp.js                   237.83 KB    841ms

Related to #1560
https://github.com/actions/toolkit/pull/1561

Start by bundling @actions/http-client with Parcel and compare the reported output before and after the recent versions. Inspect how @actions/http-client and packages such as @actions/core include undici, using the setup-cpp bundle as the size reference. Done means dependencies are tree-shakable and lazy-loaded, with the bundle no longer including the reported undici bulk.