plugin/discovery: ignore non-files when discovering
plugin/discovery: ignore non-files when discovering

If we encounter something that isn't a file -- for example, a dangling
symlink whose referent has been deleted -- we'll ignore it so that we
can either later produce a "no such plugin" error or auto-install a plugin
that will actually work.

hashicorp/terraform-plugin-sdk,hashicorp/terraform-plugin-sdk