Drop meta_requires, allow version pinning in *_requires fields

[ncoghlan]

The idea of disallowing version pinning in the *_requires fields (except for meta_requires) adds complexity for tool developers implementing the spec, and provides a worse experience for users that need to deal with dependencies with poor backwards compatibility records, or want to create a "meta package" that makes it easy to download specific versions.

The simplest solution to both problems is to just drop meta_requires and allow version pinning in the publication metadata.