Support bazel "relaxed" semver

[pombredanne]

This should be supported here, and in the VERS spec, following the addition of Bazel at:

• propose bazel type for Bazel modules package-url/purl-spec#317

See https://bazel.build/external/module#version_format

Version format

Bazel has a diverse ecosystem and projects use various versioning schemes. The most popular by far is SemVer, but there are also prominent projects using different schemes such as Abseil, whose versions are date-based, for example 20210324.2).

For this reason, Bzlmod adopts a more relaxed version of the SemVer spec. The differences include:

SemVer prescribes that the "release" part of the version must consist of 3 segments: MAJOR.MINOR.PATCH. In Bazel, this requirement is loosened so that any number of segments is allowed.
In SemVer, each of the segments in the "release" part must be digits only. In Bazel, this is loosened to allow letters too, and the comparison semantics match the "identifiers" in the "prerelease" part.
Additionally, the semantics of major, minor, and patch version increases are not enforced. However, see compatibility level for details on how we denote backwards compatibility.

Any valid SemVer version is a valid Bazel module version. Additionally, two SemVer versions a and b compare a < b if and only if the same holds when they're compared as Bazel module versions.