[Feature Request] Import deconstruction (& aliasing/macros)

Deconstruction is a quality of life feature that a lot of languages have nowadays. Since Pkl lacks the ability to alias imported functions and structures, it results in a lot of repetitive `ProprietaryServiceConfig.ReusedClassThing` or extending a class without adding anything so it can be referenced without explicitly specifying the module. This gets quite messy when different classes result in different output and the end-user file is expected to cast as one or another.

An implementation would essentially let someone do an [ES-like](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import) deconstructed import like this:

```pkl
// Provides `convertToHCL` and `Terraform`
import "/path/to/file.pkl" as { convertToHCL, Terraform }
// Provides `Terraform`, and the module as `file`
import "/path/to/file.pkl" as file, { Terraform }
```

It would also be handy to be able to write aliases or macros within a pkl file, in a similar way to how typealiases currently work. I'm primarily interested in making it easier to repeatedly reference module exports but it could also be useful for other things as well.

```pkl
import "/path/to/file.pkl"
alias convertToHCL = file.convertToHCL

output {
value = convertToHCL(someKey)
}
```

The issue names no files, tests, or entry points. Start by locating the implementation of imports and existing typealiases, then compare their behavior with the proposed deconstruction and alias forms. Done means the requested syntax and its interactions are defined and covered for the examples and invalid cases.

Written by the indexing model from the issue text.