Allow leading `swift-tools-version` in `Package.swift` for Licensing check

[dempseyatgithub]

By convention the first line of a Package.swift manifest is a comment containing the tools version:

"The version is specified on the first line of the manifest with the comment // swift-tools-version: and the version for the Swift compiler."

-Swift Package Manager Documentation

Following this convention causes the license header check of the soundness workflow to fail, even if the tools version is followed by a valid license header.

The approach I have seen taken to work around this, is to add the Package.swift file to the .licenseignore file to allow this test to pass.

Since Package.swift files are almost always present in swiftlang repositories, having license checking automatically work with the documented comment on the first line would allow these files to be more easily checked (or checked at all, instead of ignored).

This should also be allowed in version-specific manifest files such as Package@swift-5.10.swift.

Ideally the check would be able to accept both:

• First line is swift-tools-version comment
• License begins on second line

AND

• First line is swift-tools-version comment
• Second line is an empty newline
• License begins on third line

Allowing the tools version to be separated from the license by an empty newline makes it clear the tools version is separate from the license and makes both more easily readable.

[FranzBusch]

We should probably just ignore the Package.swift and the versioned manifests by default in the license check