Support for a portable haddocks directory when indexing local projects

[bmjames]

• hoogle generate --haddock=path/to/doc works similarly to hoogle generate --local, getting package info from a local ghc-pkg database, but uses path/to/doc as the canonical source of Haddock documentation. path/to/doc should be a path to the directory that contains the root index.html generated by cabal haddock or stack haddock. Paths stored in the database are relative to this root.

• hoogle server --haddock=path/to/doc is intended to be used with a database containing relative paths. In contrast to hoogle server --local, paths relative to the filesystem root are not allowed; only files from path/to/doc are served.

These features are intended to be used together, when you want to create a Hoogle index for your project in one place, then serve it somewhere else. This enables a workflow such as the following:

1. Run cabal haddock or stack haddock on your build server
2. Run hoogle generate --haddock=path/to/doc on your build server (this assumes you know where the docs were written)
3. Deploy the Hoogle database and Haddock files to your documentation server & run hoogle server --haddock=new/path/to/doc
4. ???
5. Profit

[ndmitchell]

Thanks for the patch. Hopefully step 4 is release Hoogle 5.0.9, which I've just done, and tomorrow you can proceed on to step 5.