Cannot install pre-release versions of plugins

```console
$ adapt install adapt-contrib-text@4.2.0-alpha.4
<info> Getting plugin info 100% complete
<info> Finding compatible versions 100% complete
<info> Checking constraints 100% complete
<info> Installing plugins 100% complete
? Update the manifest (adapt.json)? No
The following plugins were skipped:
adapt-contrib-text
```