Speed up 'closure' computation in `ghc-pkg`

Closes #27275 (closed)

The profile has been obtained using default flavour, and the package db has 1800 entries.

> /home/hugin/Documents/haskell/ghc-homes/ghc/_default/stage1/bin/ghc-pkg -f ~/.local/state/cabal/store/ghc-10.1.20260513-inplace/package.db/ list | wc -l
1762

Before this commit:

> hyperfine '/home/hugin/Documents/haskell/ghc-homes/ghc/_default/stage1/bin/ghc-pkg -f ~/.local/state/cabal/store/ghc-10.1.20260513-inplace/package.db/ list'
Benchmark 1: /home/hugin/Documents/haskell/ghc-homes/ghc/_default/stage1/bin/ghc-pkg -f ~/.local/state/cabal/store/ghc-10.1.20260513-inplace/package.db/ list
  Time (mean ± σ):     394.8 ms ±  12.9 ms    [User: 340.4 ms, System: 47.2 ms]
  Range (min … max):   377.7 ms … 408.8 ms    10 runs

After this MR:

> hyperfine '/home/hugin/Documents/haskell/ghc-homes/ghc/_default/stage1/bin/ghc-pkg -f ~/.local/state/cabal/store/ghc-10.1.20260513-inplace/package.db/ list'
Benchmark 1: /home/hugin/Documents/haskell/ghc-homes/ghc/_default/stage1/bin/ghc-pkg -f ~/.local/state/cabal/store/ghc-10.1.20260513-inplace/package.db/ list
  Time (mean ± σ):     186.4 ms ±   4.9 ms    [User: 136.1 ms, System: 45.2 ms]
  Range (min … max):   179.2 ms … 198.8 ms    16 runs