- Packages can remove the top-level `url` attribute and still work
- These are now legal:
- Packages with *only* version-specific URLs (even with gaps)
- Packages with a top-level git/hg/svn attribute and `version`
directives for that.
- If a package has both a top-level hg/git/svn attribute AND a top-level
url attribute, the url attribute takes precedence.