validation fixes

[matt-phylum]

Overview

This PR tightens the validation on subpaths and package types.

Subpaths are sanitized when calling with_subpath to match the formatting rules: https://github.com/package-url/purl-spec/blob/7f7e82f73c38a4a339f88abf1f2aa031e9c7af23/PURL-SPECIFICATION.rst?plain=1#L313-L323

Subpaths with %2E segments are handled the same as . segments: https://github.com/package-url/purl-spec/blob/7f7e82f73c38a4a339f88abf1f2aa031e9c7af23/PURL-SPECIFICATION.rst?plain=1#L339-L347 (changed by package-url/purl-spec#394)

When using the string shapes instead of PackageType, the package type validation is changed to require that the type begins with a letter: https://github.com/package-url/purl-spec/blob/7f7e82f73c38a4a339f88abf1f2aa031e9c7af23/PURL-SPECIFICATION.rst?plain=1#L136

Checklist

• Does this PR have an associated issue?
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• Have you created sufficient tests?

Issue

What issue(s) does this PR close. Use the closes #<issueNum> here.

[Allan-Clements]