Fix haddock source links and hyperlinked source

There were a few issues with the hackage links:

1. We were using the package id rather than the package name for the
   package links. This is fixed by now allowing the template to mention
   %pkg% or %pkgid% and substituing both appropiatly.
2. The `--haddock-base-url` flag is renamed to `--haddock-for-hackage`
   as the new base link works on a local or remote hackage server.
3. The "src" path including too much stuff, so cross-package source
   links were broken as the template was getting double expanded.

Fixes #24086