Better support for large project config files

Please help us maintain our evergreen project file

Cloud's MMS evergreen.yml is now 7.7k lines - this makes maintaining it very painful. Please enable us to split it into multiple files without switching everything to dynamic tasks.

Easy idea: supply multiple yaml files in Evergreen project config, they get read in order, last-definition-wins.
If this doesn't sound good, there are various other yaml non-standards on injection and transclusion.