fix(plugin): use compiler.webpack when possible

[sevilyilmaz]

Applies the fix (f7ee30b) that resolved the same issue on v15.

Context

When there are different versions of Webpack used in a repo (for example, one is added to the repo and the other one is coming from a node module) Webpack loses its reference to the Compiler object. The plugins no longer need to require('webpack') in Webpack 5. It's recommended to use compiler.webpack where possible.

Related comments of the Webpack authors/contributors in a similar issue: webpack/webpack#12606 (comment), webpack/webpack#12606 (comment)

Additional

I had to use ts-ignore because the branch uses webpack@4 types.

Related to #1781

[haoqunjiang]

Thanks!