Fix version-specific manifest for 5.6..<5.9

[lorentey]

This fixes the typo in the name of the Swift 5.6 package manifest, and also adds explicit manifests for Swift 5.7 and 5.8.

The default manifest has to be the 5.9 one, as we want it to get automatically picked up for Swift tool versions 5.10, 5.11, 6.0, 7.5 etc.

The manifest for 5.7 and 5.8 is the same as the one for 5.6. This PR uses symlinks to link all three of them to the same source file, assuming that symlinks are now a thing on every supported platform

Resolves #92.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've verified that my change does not break any existing tests.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test