read root level composer + user configuration files
read root level composer + user configuration files

During the CI revamp, we pulled out a manual step that would copy the
`composer.json` file into the same directory as the `PluginsActions`
class. This seems quite verbose so instead of restoring that
functionality, let's just reuse the root level files.

Closes #380

cloudflare/Cloudflare-WordPress,cloudflare/Cloudflare-WordPress,cloudflare/Cloudflare-WordPress