Ignore non-json files when listing all package configs

[charlespierce]

Closes #980

Info

• Currently, the inventory::package_configs function reads all of the files in the package config directory and attempts to parse them as package configs.
• However, sometimes the OS will automatically generate hidden files in a directory (e.g. .DS_Store on macOS).
• Those auto-generated files will not be valid package configs, but they shouldn't cause volta list to fail, instead they should be ignored.

Changes

• Updated the directory walk within package_configs to ignore both directories and any file that doesn't have a .json extension.

Tested

• Locally tested to confirm that adding an empty dotfile into the package config directory does not cause volta list to fail.

[chriskrycho]

🎉 ⚡