Add a public package for xpkg specification values

[jastang]

Description of your changes

Adds a public package for constant values defined by Crossplane's xpkg specification.

ref: https://github.com/crossplane/crossplane/blob/main/internal/xpkg/name.go, which is internal.

I have:

• [x] Read and followed Crossplane's contribution process.
• [x] Run earthly +reviewable to ensure this PR is ready for review.
• [ ] Added or updated unit tests.
• [ ] Linked a PR or a docs tracking issue to document this change.
• [ ] Added backport release-x.y labels to auto-backport this PR.

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