Absolutize 'require:' paths read from wp-cli.yml

Steps to reproduce:

1. Set `require:` with a relative path in `wp-cli.yml`.
2. Run `wp` from a subdirectory.

We already "absolutize" `path:`; we should do the same for `require:`.

An additional complication is that in MAMP setups, for example, users don't even get a fatal error; they just get no output.