// loader instructs it to dynamically expand those roots to the full "all"
// pattern as loading progresses.
//
// The pkgInAll flag on each loadPkg instance tracks whether that
// package is known to match the "all" meta-pattern.
// A package matches the "all" pattern if:
// 	- it is in the main module, or
// 	- it is imported by any test in the main module, or