omit curdir from PackageLoader root path

[davidism]

PackageLoader removes the current directory path segment after calling normpath. This fixes compatibility with loading from the root of zip imports.

• fixes PackageLoader fails loading from zip/egg when package_path="" #1467

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.