fix: check if classes are already loaded before attempting to autoload them.

[justlevine]

What

This PR updates the Autoloader to check if our main GF class is already loaded, before attempting to load the autoloader

Why

If the entire package is loaded as a dependency, it's possible the autoloader will be generated higher up in the tree.

Fixes #439

How

Instead of coupling this to Composer, we use a class_exists() check.

Testing Instructions

Additional Info

Checklist:

• [x] This PR is tested to the best of my abilities.
• [x] This PR follows the WordPress Coding Standards.
• [x] This PR has proper inline documentation.
• [x] This PR has unit tests to verify the code works as intended.
• [x] The changes in this PR have been noted in CHANGELOG.md

[coveralls]

Pull Request Test Coverage Report for Build 10222692586

Details

• 0 of 2 (0.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.02%) to 84.035%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Autoloader.php	0	2	0.0%
<!--	Total:	0	2	0.0%	-->

Totals
Change from base Build 9535520304:	-0.02%
Covered Lines:	6390
Relevant Lines:	7604

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💛 - Coveralls