Determine correct PURL for top-level package in Swift parser

The PURL for a Swift package depends on where the package is hosted, and given that `Package.swift` does not contain the repository URL for the top-level package, it is not possible to infer the correct PURL for the top-level package solely from the `Package.swift` manifest.
We can use https://swiftpackageindex.com/ to get the correct PURL, but this may not work for all packages since Swift Package Index is not authoritative in nature, i.e. a Swift package may exist, and the author may choose not to add their package to Swift Package Index. https://swiftpackageindex.com/add-a-package

This is a follow from this discussion https://github.com/nexB/scancode-toolkit/pull/3788#discussion_r1618829825