v0.48.0

Most relevant commits:

• native: use 'Declaration' type for a declaration
• compiler/checker: make clearer an error message loading a package
• native: revert 'make 'MapPackage' methods with pointer receivers'
• native: rename 'Package.Name' method to 'PackageName'
• native: fix 'CombinedPackage.PackageName' method documentation
• compiler/checker: fix import of native packages with not allowed names
• native: replace 'Package.DeclarationNames' method with 'LookupFunc'