`stack haddock` should use precompiled packages if they have haddocks

#### Steps to reproduce:
1. Create a project and note the LTS snapshot used by its resolver.
2. Add some dependency to the `.cabal` file that has the same version between the the project's snapshot and an older snapshot.
3. Run `stack --resolver= haddock`
4. Run `stack haddock`
#### Expected behaviour:

Since haddocks exist for the precompiled package from the older snapshot, the precompiled package should be copied.
#### Actual behaviour:

The package is rebuilt.
#### Notes:

[this code](https://github.com/commercialhaskell/stack/blob/7915fca986fcaf76dee218709fec6bbdee3e83a0/src/Stack/Build/Execute.hs#L944) triggers the haddock compilation. However, that cannot just be removed because detection of whether the precompiled package already has haddocks does not work.