code: func (t Expander) Expand(template string) (pkg string, err error) {
  	return t.expandImpl(template, false)
  }
docs: // Expand applies package template expansion rules to the package template,
  //
  // If err == ErrSkipTemplate, that means that this template does not apply to
  // this os/arch combination and should be skipped.
  //
  // The expansion rules are as follows:
  //   - "some text" will pass through unchanged
  //   - "${variable}" will directly substitute the given variable
  //   - "${variable=val1,val2}" will substitute the given variable, if its value
  //     matches one of the values in the list of values. If the current value
  //     does not match, this returns ErrSkipTemplate.
  //
  // Attempting to expand an unknown variable is an error.
  // After expansion, any lingering '$' in the template is an error.