Java: Add Import.getATypeImport

When working with Javadoc it is often necessary to find out to which type a Javadoc tag refers to. Imports are one way to find this out, however the QL `Import` class does not define a common predicate to get an imported type despite all import types being able to import types in some way.
It would therefore be useful to add a predicate `Import.getATypeImport()` which would behave like this:
- `ImportType`: `getImportedType()`
- `ImportStaticTypeMember`: `getATypeImport()`
- `ImportOnDemandFromPackage`: `getAnImport()`
- `ImportOnDemandFromType`: `getAnImport()`
- `ImportStaticOnDemand`: `getATypeImport()`

Edit: As described by #10498 instead of adding this predicate, it might make more sense to have a predicate for `CompilationUnit` which obtains a type which is available by its simple same in that compilation unit.