Support autoload.php from WP root

[chellman]

I've worked around this issue (originally mentioned here) in other ways, and assumed based on previous discussions that I must have something misconfigured or am doing something wrong. But I've just spun up a new site with Carbon Fields, everything in standard locations, and I'm convinced that I might not be the only one, and should submit a PR.

Here's the problem: I'm using CF as a plugin, installed with composer as it's supposed to be. The directory structure is standard WP, with these relevant pieces:

/wp-content/plugins/carbon-fields-plugin
/wp-content/plugins/my-custom-plugin
/vendor/autoload.php /composer.json

There's nowhere else in the codebase attempting to load the autoload.php file, because CF is currently the only composer-installed piece. The CF plugin is supposed to fill that need as I understand it, but it only checks for the autoload file in its own directory via DIR, which doesn't working here. So I've added a conditional to look for the vendor/autoload file in the WordPress root, which seems like a reasonable fallback to include.

Am I missing something about the way composer should be working that this is a bad choice?