Compatibility layer: define constants earlier?

[rbackert]

Constants may need to be defined earlier than the plugins_loaded action. Incompatible plugins may define the constants the Pantheon Compatibility plugin is meant to fix during their plugin load, effectively overriding Pantheon's attempt to define the constant.

I observed this in a local lando environment with WP Rocket, which defines the constants noted in Pantheon's compatibility guide (WP_ROCKET_CONFIG_PATH, WP_ROCKET_CACHE_ROOT_PATH, etc.) during it's plugin load, before Pantheon Compatibility tries to define them during plugins_loaded.