cmd/go/internal: factor out modload.QueryPackage and use in in modget
modload.Import contains a loop that looks for the module containing a package.
Because we overload Import to locate both packages and modules, that loop
contains a bunch of special-cases for modules with empty roots.
In this change, we factor out the loop into a new function (QueryPackage) and
use that directly in modget.getQuery. That restores the invariant that
the paths passed to modload.Import must be importable packages, and fixes 'go
get' lookups for packages that have moved between a module and submodules with
the same path prefix.