Rewrite Documentation/Source links on hover to use local hoogle/hackage if available on nix and otherwise

## Is your enhancement request related to a problem? Please describe.

This request is similar to #208, but slightly different? At least the fact that this issue is focused on nix support makes it different, but the implementation of this could satisfy both issues I believe.

Even when I have a local instance of hoogle running at `localhost:8888` the links I click for source and documentation go to `http://haskell.org`.

NOTE: I use nix's `withHaddock` feature for this

For instance I'm linked to:

https://hackage.haskell.org/package/apecs-0.9.4/docs/Apecs-Util.html#v:newEntity_

When I'd prefer to be linked to:

http://localhost:8888/file/nix/store/pmyb1ayz7vxlmim1bd12sjpss3s7pddy-apecs-0.9.4-doc/share/doc/apecs-0.9.4/html/Apecs.html#v:newEntity_

## Describe the solution you'd like

For HLS to check to see if a hoogle instance is running and if it's fingerprint is a nix server, rewrite urls like:

```diff
- https://hackage.haskell.org/package/apecs-0.9.4/docs/Apecs-Util.html#v:newEntity_
+ http://localhost:8888/file/nix/store/pmyb1ayz7vxlmim1bd12sjpss3s7pddy-apecs-0.9.4-doc/share/doc/apecs-0.9.4/html/Apecs.html#v:newEntity_
```

And for source:

```diff
- https://hackage.haskell.org/package/apecs-0.9.4/docs/src/Apecs.Util.html#newEntity_
+ http://localhost:8888/file/nix/store/pmyb1ayz7vxlmim1bd12sjpss3s7pddy-apecs-0.9.4-doc/share/doc/apecs-0.9.4/html/src/Apecs.Util.html#newEntity_
```

Looking at the above, the common replacement *within* the urls is:

```diff
- /package/apecs-0.9.4/docs
+/file/nix/store/pmyb1ayz7vxlmim1bd12sjpss3s7pddy-apecs-0.9.4-doc/share/doc/apecs-0.9.4/html
```

You can get this variable in recent versions of Nix's `haskellPackages` from the environmental variable `$NIX_GHC_DOCDIR`.

*NOTE*: I believe the code to change is in [Development.IDE.Spans.Documentation.getDocumentationsTryGhc](https://github.com/haskell/haskell-language-server/blob/53f61b4fa9182879614d33ccd74c2242c32edb4c/ghcide/src/Development/IDE/Spans/Documentation.hs#L104)

## Describe alternatives you've considered

A workaround described [here](https://github.com/haskell/haskell-language-server/issues/208#issuecomment-660701983):

> A possible workaround is to active documentation generation for all dependant packages adding in the cabal.project:

```
package *
documentation: true
```

To be frank though, I'd just never do that and probably forget it was ever an option.