As a companion to WP_CLI_EARLY_REQUIRE but doesn't include the files
until after wp has been through the boostrap process. This is the same
as using `--require` from the command line, or require in a yaml config
file. The purpose for another way of requiring a file is for use in
codecov testing, where it otherwise is not practical to conditionally
including a php file at normal runtime using the existing methods in a
testing environment.