docs: def package_version(self, prefix=None, name=None, pkg=None, build=False):
          """Get installed package version in a given env."""
          package_versions = {}
  
          if name and prefix:
              raise TypeError("Exactly one of 'name' or 'prefix' is required.")
  
          if name:
              prefix = self.get_prefix_envname(name)
  
          if self.environment_exists(prefix=prefix):
  
              for package in self.linked(prefix):
                  if pkg in package:
                      n, v, b = self.split_canonical_name(package)
                      if build:
                          package_versions[n] = '{0}={1}'.format(v, b)
                      else:
                          package_versions[n] = v
  
          return package_versions.get(pkg)