Handle Swift registry identities

[andrew]

Swift Package Registry identities do not include source repository coordinates, so treating apple/swift-argument-parser as a namespace creates a misleading PURL.

Return unsupported results for Swift registry identities and build source PURLs only from coordinates such as github.com/apple/swift-argument-parser. This also keeps the struct and string construction paths aligned.

Closes #28

[Copilot]

Pull request overview

Updates Swift PURL handling to avoid fabricating source-based Swift PURLs from Swift Package Registry identities, returning “unsupported” outputs instead, and keeps the struct-based and string-based construction paths aligned.

Changes:

• Refactors namespace/name extraction into splitNamespace and adds an ok return to represent unsupported identifiers.
• Adds Swift-specific parsing to accept only source-coordinate-style identifiers (host/owner/package) and reject registry identities/paths.
• Adds/updates tests to cover Swift source coordinates and registry identities across MakePURL* and BuildPURLString.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
makepurl.go	Adds ok-returning splitNamespace, uses it in BuildPURLString, and introduces Swift-specific identifier validation.
makepurl_test.go	Adds BuildPURLString tests for Swift source coordinates vs registry identities/paths and aligns parity test coverage.
ecosystem.go	Routes MakePURL / MakePURLString through splitNamespace, returning nil/empty string for unsupported Swift registry identities.
ecosystem_test.go	Adds Swift source coordinate tests and explicit rejection tests for Swift registry identities for both struct and string APIs.

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