Fix some issues reported by lintian

Fix some issues reported by lintian

• Use correct machine-readable copyright file URI. (out-of-date-copyright-format-uri)
• Update watch file format version to 4. (older-debian-watch-file-standard)

These changes have no impact on the binary debdiff.

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/access-modifier-checker/555f6f55-4d14-46a3-9906-29595ed89b81.