This PR adds the ability for incremental builds when using the Webpack
Plugin.
Instead of using the source library directly, you can now utilize the
output folder by utilizing the `buildLibsFromSource` option within your
webpack.config file, through `NxAppWebpackPlugin`. This means that
instead of accessing `mylib/src/index.ts`, it will access
`dist/mylib/index.js`.
This directly aligns with incremental builds as it ensures that the
build process only recompiles the source doe that has been modified
since the last build.
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