Implement nested types

I'd like to be able to do something like this:

```pkl
class MyA {
properties: Properties

class Properties {
prop1: String
}
}

class MyB {
properties: Properties

class Properties {
prop2: String
}
}

aProps: MyA.Properties
```

Currently, this requires disambiguating classes with only naming, which can lead to very long class names with more deeply nested structures. This code today must look like this:
```pkl
class MyA {
properties: MyAProperties
}

class MyAProperties {
prop1: String
}

class MyB {
properties: MyBProperties
}

class MyBProperties {
prop2: String
}

aProps: MyAProperties
```

The issue names no files, tests, or entry points. Start by locating the compiler areas that parse and resolve class declarations and qualified type references; done means the nested class examples compile and MyA.Properties resolves distinctly from MyB.Properties.

Written by the indexing model from the issue text.