Fixing the cache situation

[Anton-Latukha]

Because the number of small things are addressed here - it is easier to make documentation in commits & address them in review mode.

[Anton-Latukha]

Current source caching works great:

      - name: Hackage sources cache
        uses: actions/cache@v2
        env:
          cache-name: hackage-sources
        with:
          path: ${{ env.CABAL_PKGS_DIR }}
          #  2021-12-01: NOTE: Cabal Hackage source tree storage does not depend on OS or GHC.
          key:          ${{ env.cache-name }}-${{ env.HACKAGE_TIMESTAMP }}
          restore-keys: ${{ env.cache-name }}-

All GHCs use one 193Mb cache instance & none of them needed to download additional versions.

But depending on the library versions supporting particular base - some package downloads can happen for particular GHCs here & there down the line in the future.

The most ideal way to cache is to differentiate on GHC versions:

      - name: Hackage sources cache
        uses: actions/cache@v2
        env:
          cache-name: hackage-sources
        with:
          path: ${{ env.CABAL_PKGS_DIR }}
          #  2021-12-01: NOTE: Cabal Hackage source tree storage does not depend on OS or GHC really, but can depend on `base`.
          key:          ${{ env.cache-name }}-${{ matrix.ghc }}-${{ env.HACKAGE_TIMESTAMP }}
          restore-keys: |
            ${{ env.cache-name }}-${{ matrix.ghc }}-
            ${{ env.cache-name }}-

But that means storing (6 GHCs) - 6 times more sources. So using 10% of the cache for it.

Source caching is really to be done inside master branch for caching & that gets shared into PRs.
But PRs really would not change or even use dependency sources, since they would have & (should) use dependencies compiled.

And if the person updates the Hackage pin - it anyway needs full redownload & rebuild, of caches also.

So source caching is really only happens in master & only for master having a faster compilation of deps. In that way it is revealed that occasional partial redownload from Hackage is a redundant theme to drive attention to & discuss & that source caching at all can be removed without notacible implications.

I would keep it as it is & would put a statement to "think hard - that is a master for master purposes".

[Anton-Latukha]

Closing in favour of #2446 continuation.