[entgql] Generate multiple sources

[cliedeman]

EarlySourceInjector allow caters for injecting a single graphql file.
I would like to be able to generate multiple for easier debugging.

E.g.
relay.graphql, enums.graphql, connection1.graphql. etc.

I guess this could also be done as a new plugin to avoid breaking backwards compatibility

type EarlySourcesInjector interface {
	InjectSourcesEarly() ([*ast.Source], error)
}