`tea postgres@500 --version` executes 15.2

This looks more like a bug than a feature to me.

```shell
$ ./tea postgres@500 --version
postgres (PostgreSQL) 15.2
$ ./tea postgres@300.2 --version
resolving package graph
error: not-found: pkg: postgresql.org~300.2

inventory: 11.19.0, 12.14.0, 13.1.0, 14.7.0, 15.2.0

$ ./tea postgres@13.9 --version
installed: ~/.tea/postgresql.org/v13.1.0
postgres (PostgreSQL) 13.1
```