Expand ${pkgroot} variable when reading haddock-html field from ghc-pkg

[mpickering]

This fixes #11217

No test, as it is difficult to test since you need to:

1. Ensure there is a package registration with ${pkgroot}
2. Check the existence of a certain link in the generated HTML.

Manual QA: Inspect the output of running cabal haddock on a package and notice the links to base documentation work.

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[geekosaur]

I would explain why here (that is, mention ${pkgroot}).

I'm not sure if this might count as significant. Artem/others?