hadrian: docs-haddock target is a big hammer, make it more fine-grained

If I am working on the haddock documentation for a particular package then I just want to rebuild haddocks for that package.

It would be convenient if docs-haddock could take an additional filter argument to specify which package I want haddocks built for.

For example

docs-haddock:template-haskell will just build the haddock documentation for the template-haskell library.

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