cmd/go/internal/load: pass the importer's package path when checking visibility
A module like "gopkg.in/macaroon.v2" might have a test with a "_test" package
suffix (see https://golang.org/cmd/go/#hdr-Test_packages).
When we compile that test, its ImportStack entry includes the "_test" suffix
even though nothing else can actually import it via that path.
When we look up the module containing such a package, we must use the original
path, not the suffixed one.
On the other hand, an actual importable package may also be named with the
suffix "_test", so we need to be careful not to strip the suffix if it is
legitimately part of the path. We cannot distinguish that case by examining
srcDir or the ImportStack: the srcDir contaning a module doesn't necessarily
bear any relationship to its import path, and the ImportStack doesn't tell us
whether the suffix is part of the original path.
Fortunately, LoadImport usually has more information that we can use: it
receives a parent *Package that includes the original import path.