Support build-tool-depends

[amesgen]

It would be nice if build-tool-depends could be supported, especially for compiler plugins like autoexporter or tasty-discover. In particular, running haskell-language-server-wrapper manually fails as the required binaries are not in scope. This can be fixed manually by placing the binaries in PATH, but that contradicts using build-tool-depends (at least with nix-style builds) in the first place.

Example

git clone --single-branch --branch hls-build-tool-depends https://github.com/amesgen/stuff.git && cd stuff

Then

haskell-language-server-wrapper

fails with

File:     /tmp/tmp.omchVSmVF6/stuff/app/Words.hs
Hidden:   no
Range:    1:1-2:1
Source:   compiler
Severity: DsError
Message:  haskell-language-server-8.10.3: could not execute: autoexporter
[INFO] finish: User TypeCheck (took 0.03s)
Files that failed:
 * /tmp/tmp.omchVSmVF6/stuff/app/Main.hs
 * /tmp/tmp.omchVSmVF6/stuff/app/Words.hs

Completed (2 files worked, 2 files failed)

but

cabal exec -- haskell-language-server-wrapper

works just fine.

[amesgen]

Duplicate of #176 (I only searched for "build-tool-depends"...)