Chunk dependencies not flushed when it's moved to a parent

There's this flag [`reuseExistingChunk` of SplitChunksPlugin](https://webpack.js.org/plugins/split-chunks-plugin/#splitchunks-cachegroups-cachegroup-reuseexistingchunk) which, when `true`, moves a chunk up to the parent cache group to avoid outputting duplicated code, and doesn't generate any file for the source chunk itself. This conflicts with the behaviour of `flushChunkNames()` since it won't look for chunk dependencies inside `stats.namedChunkGroups` if it's not inside `stats.chunks` in the first place, but it should. More people might face this same issue because it's the default behaviour of `splitChunks.cacheGroups.defaults`.

To be honest, I'm not 100% sure if it's because of `reuseExistingChunk`. Maybe this is just the expected normal behaviour.

Scenarios:

* User has defined a cache group but it only encompasses one module, meaning the group chunk will be generated but not a chunk for the source module
* User has defined a cache group but, because of restrictions provided by *minChunks*, *minSize*, *maxSize* or any other config parameter, Webpack ends up moving a module to the parent chunk

Related code fragment: https://github.com/faceyspacey/webpack-flush-chunks/blob/b3534a330096b40912a0735e518b047563aa61bf/src/flushChunks.js#L233-L238