// ignoredByGoTool reports whether the given import path corresponds
// to a directory that would be ignored by the go tool.
//
// The logic of the go tool for ignoring directories is documented at
// https://golang.org/cmd/go/#hdr-Package_lists_and_patterns:
//
//      Directory and file names that begin with "." or "_" are ignored
//      by the go tool, as are directories named "testdata".
//
// However, even though `go list` and other commands that take package
// wildcards will ignore these, they can still be imported and used in
// working Go programs. We continue to ignore the "." and "testdata"
// cases, but we've seen valid Go packages with "_", so we accept those.