When a constraint is defined including build metadata, go-version ignores
this, and if a user explicitly requests a build version, this will now return that version.
Also covers cases when a user adds build metadata to a non-equality constraint,
or in a constraint with multiple conditions (ex. >0.8.0+abc, <1.0.0), and gives
and error when this occurs.
Adds basic detector for registry module source strings. While this isn't
a thorough validation, this will eliminate anything that is definitely
not a registry module, and split out our host and module id strings.
lookupModuleVersions interrogates the registry for the available
versions of a particular module and the tree of dependencies.
Pam Selle