"latest" packages

Provide a way to use the "latest" version of a package, where, the version is queried during eval.

Imagined syntax:

```pkl
import "package://example.com/foo@1.+#/myModule.pkl"
```

Or, when as a dependency:

```pkl
amends "pkl:Project"

dependencies {
["foo"] { uri = "package://example.com/foo@1.+" }
}
```

Some thoughts:

1. Projects with "latest" dependencies should be prevented from being published as packages
2. The "latest" should only be applied to the minor version or below, because the major version is part of the package identifier.

This requires a package index that can be queried.

The issue names no files, tests, or entry points. Begin by locating package dependency resolution and the package-index design, then determine how version queries and package publication are handled. Done should cover the proposed latest-version semantics, minor-version constraint, and prevention of publishing projects with such dependencies.

Written by the indexing model from the issue text.