Preserve plain versions when building PURLs

[andrew]

Preserves already-valid version strings in CleanVersion and only parses native range syntax when the input is a constraint.

Updates github.com/git-pkgs/vers to v0.6.0. This keeps Composer 1.0 unchanged with the new release while retaining minimum-bound extraction for constraints.

[Copilot]

Pull request overview

This PR updates version cleaning to preserve already-valid “plain” version strings when building PURLs, only invoking native constraint parsing when the input is a constraint (using vers), and bumps github.com/git-pkgs/vers to v0.6.0 to support Composer 1.0 pass-through.

Changes:

• Trim and short-circuit CleanVersion to return already-valid versions unchanged via vers.ValidWithScheme.
• Add test cases to ensure plain versions like 1.0 for Composer/npm pass through unchanged.
• Update dependency github.com/git-pkgs/vers to v0.6.0 (and corresponding go.sum entries).

Reviewed changes

Copilot reviewed 3 out of 4 changed files in this pull request and generated 2 comments.

File	Description
makepurl.go	Trims input and preserves already-valid versions before attempting constraint parsing.
makepurl_test.go	Adds pass-through test coverage for plain versions (Composer/npm).
go.mod	Bumps github.com/git-pkgs/vers to v0.6.0.
go.sum	Updates module checksums for vers v0.6.0.

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