Race condition around `me.fornever.haskeletor.external.component.StackProjectManager#haddockBuilding`

We have an issue in the `StackProjectManager`: currently its clients check the value of `haddockBuilding` flag sometimes, and will cancel updates in case when the flag is `true`.

Imagine that we start two simultaneous activities somehow: the first will check the flag, the second will check the flag. The first will set it to `true`, the second the same. Then, they will execute simultaneously.

We should introduce some merging queue for these builds or something.

See the code near this line: https://github.com/ForNeVeR/haskeletor/blob/2620e1ad5c446ac9c6a7b756ee6d4d593e86e340/stack/src/main/kotlin/me/fornever/haskeletor/stack/hoogle/HoogleBuilder.kt#L65

Also, look for the number of this issue in the project code base.