It is possible to enter the parent-walking directory loop in a way that
it will loop forever - if mdir is empty, and d reaches ".". To avoid
this, make sure that the 'd = filepath.Dir(d)' step only happens if the
parent directory is actually different than the current directory.
This fixes some of the tests like TestImport/golang.org_x_net_context,
which were never finishing before.
While at it, also fix TestImport/golang.org_x_net, which seems to have
the wrong expected error. The root of the x/net repo doesn't have a
go.mod file, nor is part of a module itself, so it seems like the
expected error should reflect that.
After these two changes, 'go test cmd/go/internal/modload' passes on my
linux/amd64 machine.