Importation of file is creating a public: directory

Problem/Motivation

When a file uri is defined to exist directly under "public://" directory, there is a weird behavior. A "web/public:" directory is being created!

Steps to reproduce

# Create an article with a photo "photo.jpg";
# Export the content;
# Manually edit the exported yaml file for the file to have the uri with value "public://photo.jpg";
# Install from scratch and enable default_content to have the importation running;
# Check that a "web/public:" directory was created.

Proposed resolution

Use the drupal dirname method instead of the PHP dirname. This alone will fix the problem.