fix egde case where a HMR chunk is incorrectly downloaded

[sokra]

fix egde case where a HMR chunk is incorrectly downloaded when loading a unchanged chunk during HMR downloading

What kind of change does this PR introduce?
bugfix

Did you add tests for your changes?
yes

Does this PR introduce a breaking change?
no

What needs to be documented once your changes are merged?
nothing

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[tmeasday]

@sokra I applied this patch to the original issue described in #15541

What I observed:

• In the reproduction repo (where we trigger a bug via hitting multiple route w/ playwright) it didn't really make much difference.

• In an actual larger app, where you can trigger the issue by quickly changing "routes" (components in storybook), the behaviour changed (probably you'd say improved) but didn't resolve:

  • Previously (on the lazy-compilation branch, if you change routes quickly (run yarn storybook, then in the browser, use the cmd-down shortcut repeatedly to jump between components), you would eventually see a 404 which bubbles up as an error from import():

• With the patch applied (on the lazy-compilation-webpack-15586 branch), the same action leads to eventually the import() just hanging, and this error appearing in the console:

[sokra]

Yeah, there were some obvious bugs in my PR. I hope they are fixed now...