1. CI caches 3rd patry dependency binaries.

2. That is why we do not want to trigger on all `.cabal` changes. We want to
cache only when changes in `.cabal` descriptions lead to changes in versions of
deps used, or in their compilation flags. `freeze` files:

For example the bits of the resulting `.freeze` file:

active-repositories: hackage.haskell.org:merge
constraints: any.Boolean ==0.2.4,
             ...
             any.HsYAML ==0.2.1.0,
             HsYAML -exe,
             any.HsYAML-aeson ==0.2.0.1,
             HsYAML-aeson -exe,
             any.JuicyPixels ==3.3.6,
             JuicyPixels -mmap,
             any.MemoTrie ==0.6.10,
             MemoTrie -examples,
             ...
             any.QuickCheck ==2.14.2,
             QuickCheck -old-random +templatehaskell,
             aeson -bytestring-builder -cffi -developer -fast,
             ...
             any.aeson-pretty ==0.8.9,
             aeson-pretty -lib-only,
             bifunctors +semigroups +tagged,
             any.blaze-textual ==0.2.2.1,
             blaze-textual -developer -integer-simple +native,
             ...
             any.brittany ==0.13.1.2,
             brittany -brittany-dev-lib -brittany-test-perf,
             ...
             any.clock ==0.8.2,
             clock -llvm,
             any.cmdargs ==0.10.21,
             cmdargs +quotation -testprog,
             ...
             any.comonad ==5.0.8,
             comonad +containers +distributive +indexed-traversable,
             ...
             any.hashtables ==1.2.4.2,
             hashtables -bounds-checking -debug -detailed-profiling -portable -sse42 +unsafe-tricks,
             haskell-language-server -alternatenumberformat +brittany +callhierarchy +class +eval +floskell +fourmolu +haddockcomments +hlint -ignore-plugins-ghc-bounds +importlens +modulename +ormolu -pedantic +pragmas +qualifyimportednames +refineimports -rename +retrie +splice +stylishhaskell +tactic,
             ...
             any.hlint ==3.2.7,
             hlint -ghc-lib +gpl -hsyaml +threaded,
             hls-eval-plugin -pedantic,
             hls-graph -embed-files -pedantic -stm-stats,
             hls-hlint-plugin -ghc-lib -hlint33 -pedantic,
             ...
             any.integer-logarithms ==1.0.3.1,
             integer-logarithms -check-bounds +integer-gmp,
             ...
             any.lens ==5.0.1,
             lens -benchmark-uniplate -dump-splices +inlining -j +test-hunit +test-properties +test-templates +trustworthy,
             ...
             any.scientific ==0.3.7.0,
             scientific -bytestring-builder -integer-simple,
             any.semigroupoids ==5.3.6,
             semigroupoids +comonad +containers +contravariant +distributive +tagged +unordered-containers,
             any.semigroups ==0.19.2,
             semigroups +binary +bytestring -bytestring-builder +containers +deepseq +hashable +tagged +template-haskell +text +transformers +unordered-containers,
             ...
             any.transformers-compat ==0.6.6,
             transformers-compat -five +five-three -four +generic-deriving +mtl -three -two,
             ...
             any.vector ==0.12.3.1,
             vector +boundschecks -internalchecks -unsafechecks -wall,
             any.vector-algorithms ==0.8.0.4,
             vector-algorithms +bench +boundschecks -internalchecks -llvm +properties -unsafechecks,
             ...
index-state: hackage.haskell.org 2021-11-29T12:28:17Z
```

You got the idea. `freeze` collects stuff for dependencies.

I also checked - it solves & collects all package configuration across all
subprojects in one file.

Before we not had this at all.

Now - most cases would use it.

But for example, all HLS subprojects can not solve single version of deps for
GHC 9.0 at once.

And depending on `.cabal` is not the right way, as it means to account of only 1
configuration of many in the project.

…e-vers