Package aliases

It would be nice if we could alias package imports.

What I do now:

```pkl
amends "package://github.com/aws-cloudformation/rain/releases/download/v1.8.2-alpha1/cloudformation@1.8.2-alpha1#/template.pkl"
import "package://github.com/aws-cloudformation/rain/releases/download/v1.8.2-alpha1/cloudformation@1.8.2-alpha1#/aws/s3/bucket.pkl"

Description = "testing imports"
Resources {
["Foo"] = new {
Type = "AWS::S3::Bucket"
}
["Bar"] = new bucket.Bucket {
BucketName = "my-bucket-name"
}
}
```

What I would like to do:

```pkl
use "package://github.com/aws-cloudformation/rain/releases/download/v1.8.2-alpha1/cloudformation@1.8.2-alpha1" as cfn

amends "cfn/template.pkl"
import "cfn/aws/s3/bucket.pkl"

or

amends cfn.template
import cfn.aws.s3.bucket

...

```

No files, tests, or entry points are named. Start by locating the parser and resolver for package imports, then compare the proposed `use ... as` syntax with the qualified-name alternative. Done means one defined alias form supports the demonstrated `amends` and `import` examples with corresponding tests.

Written by the indexing model from the issue text.