XO1306: import from a module xo.mod does not require

An import path whose first element is a host name (github.com/acme/billing/invoice) names a package of another module, and that module must be required in xo.mod so its version is selected and locked. xo get <module path> adds the latest version; xo mod tidy adds a requirement for every such import. See core 4.1, 4.2, and decision 0086.

Example

use github.com/acme/billing/invoice   // xo.mod has no require for it

fn total() -> Int { invoice.total() }

Fix

// xo get github.com/acme/billing adds to xo.mod:
require github.com/acme/billing v1.4.0

Run xo explain XO1306 for this text in a terminal, or see section 10.6 of the specification.