backpack crashes with an unclear error message when trying to work with digest

**Describe the bug**

I get

```
Resolving dependencies...
instantiateComponent: digest-0.0.1.2-37c2fb0db235a5f717365581080fba0fb7ee6e212351d93297f2b1e88ecaeda3
CallStack (from HasCallStack):
error, called at ./Distribution/Client/ProjectPlanning.hs:2181:21 in main:Distribution.Client.ProjectPlanning
```

when running `cabal build`

**To Reproduce**

At patch dd8d3cdb4b2950f9db57becfaf2ce3483e35b7af at my [sak repo](https://hub.darcs.net/vmchale/sak/browse/digest-sig).

Run `v2-cabal build` in any directory.

**Expected behavior**

I expect to to work the same as when I instantiate `digest-sig` with `digest-pure`, or, if there is some reason this is not possible, to have a clearer error message.

**System information**
- Linux
- cabal-install version 3.2.0.0, Cabal version 3.2.0.0, and GHC 8.2.2-8.10.1

Reproduce the failure with `v2-cabal build` at patch dd8d3cdb4b2950f9db57becfaf2ce3483e35b7af, then inspect Distribution/Client/ProjectPlanning.hs:2181. Compare instantiating `digest` with `digest-pure` and determine what should happen when using `digest-sig`; done means the build works or reports a clearer error.

Written by the indexing model from the issue text.