* Allow dismissing "releases" completely
  => allow all builds and set tags:true
* Omitting "tags" and "branch" and "asset"
  => set tags: true

Add version-constraint support for tag-based release resolution by using
normalize_version_spec and SpecifierSet when a version constraint is
present.  For unconstrained tags, keep the legacy strict semver parsing
path.

If the user uses version constraints, maybe synthesize an automatic
open-ended tags release.  This is so that the user only has to configure
frozen, left-behind version/st_build pairs while keeping the standard
newest tag for newest build semantic.

E.g.

```
    "releases": [
        {
            "sublime_text": "<4000",
            "version": "2.5.*"
        }
    ],
```

In the example, the package defines an old version for older sublime
builds.  Here we would add an open-ended release definition, e.g.

```
        {
            "sublime_text": ">=4000",
            "tags": true
        }
```

Move normalize_version_spec into normalize_release_entry for dynamic
release definitions, so constraints are canonicalized once during the
normalize phase.

This allows to treat "*" as non-constraining ("") as per docs.

When deciding whether to synthesize an automatic open-ended tags release,
handle version="*" the same as an empty/missing version constraint.

This avoids generating a synthetic >max release from definitions that are
not actually version-constrained before normalization runs.