stack haddock: Wrong index for package with sublibraries

### Steps to reproduce

For example:

1. `git clone https://git8.cs.fau.de/software/copar.git`
2. `stack haddock --open copar:lib`

### Expected

Stack generates haddocks for the unnamed library component in the cabal file and creates an index for that.

### Actual

Stack generates and shows the index for a random internal library instead of the main (exposed) one. Judging from the output, it actually generates the index for the main library but then insists to generate haddocks for the other library as well and overwrites the first index.

### Stack version

```
$ stack --version
Version 2.1.3, Git revision 636e3a759d51127df2b62f90772def126cdf6d1f (7735 commits) x86_64 hpack-0.31.2
```

### Method of installation

* Official binary, downloaded from stackage.org or fpcomplete's package repository