Ensure we load our autoloader from our plugin directory

[dkotter]

Description of the Change

In #217 a new composer dependency was added and to ensure it gets loaded, a composer autoloader was required. This was loaded with a require_once statement that just referenced the vendor directory.

The problem here is that we weren't specifically referencing the directory we should load from. This can result in the autoloader being loaded from somewhere else. For instance, if you have an autoloader at the root of your site (say you use composer to manage all your plugins), this autoloader will be loaded instead in certain situations, causing fatal errors since the dependency we expect is now not loaded.

This PR fixes this by adding the __DIR__ constant before the file path, ensuring the autoloader is loaded from the root directory of this plugin. We also output an admin notice if that autoloader isn't found and if the base Factory class doesn't exist. This should only be those that are running this plugin directly from GitHub and need to run composer install

Alternate Designs

Could use plugin_dir_path( __FILE__ ) instead of __DIR__ but they both return the same value and my preference is the latter.

Benefits

The correct autoloader is always used

Possible Drawbacks

None

Verification Process

Before checking this PR out, add a composer autoloader to the root of your WordPress install (something like wordpress/vendor/autoload.php).

Setup Restricted Site Access, add one dummy IP address that is allowed and then try and access the site from an incognito window (seems like the issue happens only if you're not logged into WordPress).

You should get a fatal error.

Then check out the code from this PR and run the same tests again.

Things should work now

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests passed.

Changelog Entry

Fixed - Ensure we load our autoloader from the root of our plugin directory
Added - Show an admin notice if our autoloader doesn't exist

Credits

Props @dkotter, @pablojmarti, @shahzaib10up

Applicable Issues

Closes #229

[peterwilsoncc]

LGTM.

Is it possible to do something like 10up/safe-svg#52 to account for people loading the plugin via composer?

I know we don't have it it packagist but it's easy enough to refer to a github repo in a top level composer file.

[peterwilsoncc]

Thank you kindly.