Restore original hpack generation logic

[srid]

Merges to #114

This restores hpack support such that it nammayatri/nammayatri#109 continues to work (PR #114 in its current form broke it¹). We probably should add another test case for a project with package.yaml only (no .cabal).

Footnotes

1. See:

          > source root is beckn-spec-0.1.0.0
          > setting SOURCE_DATE_EPOCH to timestamp 1000000000 of file beckn-spec-0.1.0.0/src/Beckn/Types/Core/Taxi/Track/Order.hs
          > patching sources
          > package.yaml: Yaml file not found: /private/tmp/nix-build-beckn-spec-0.1.0.0.drv-0/beckn-spec-0.1.0.0/package.yaml
          > /nix/store/559pz0w6zlvw8yyxah9s10fhaz400vaj-stdenv-darwin/setup: line 138: pop_var_context: head of shell_variables not a function context
          For full logs, run 'nix log /nix/store/7gxn8hmgxvzwic3gm6g5x7b74bcpw36b-beckn-spec-0.1.0.0.drv'.
   

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