Discover ghc-pkg from $LibDir/bin/ghc-pkg

```
% ghc --info|grep LibDir
,("LibDir","/opt/ghc/8.6.5/lib/ghc-8.6.5")
```

```
% ls /opt/ghc/8.6.5/lib/ghc-8.6.5/bin/ghc-pkg
/opt/ghc/8.6.5/lib/ghc-8.6.5/bin/ghc-pkg
```

I think this better than whatever path guessing we do now, and should be tried first for GHC.

cc @hvr

EDIT: looks like the `bin` might be missing for older GHCs, and the executables are directly in lib

```
/opt/ghc/7.0.4/lib/ghc-7.0.4/ghc-pkg
...
/opt/ghc/7.6.3/lib/ghc-7.6.3/ghc-pkg
/opt/ghc/7.8.4/lib/ghc-7.8.4/bin/ghc-pkg
...
/opt/ghc/8.10.1/lib/ghc-8.10.1/bin/ghc-pkg
```