hadrian: Add --haddock-base-url option for specifying base-url when generating docs

The motiviation for this flag is to be able to produce documentation
which is suitable for uploading for hackage, ie, the cross-package links
work correctly.

There are basically three values you want to set this to:

* off - default, base_url = ../%pkg% which works for local browsing
* on - no argument , base_url = https:://hackage.haskell.org/package/%pkg%/docs - for hackage docs upload
* on - argument, for example, base_url = http://localhost:8080/package/%pkg%/docs for testing the documentation.

The `%pkg%` string is a template variable which is replaced with the
package identifier for the relevant package.

This is one step towards fixing #21749

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