prost-build: Split Config into a separate module and file.

[gibbz00]

See note about possibly changing the visibility of some helper methods to private . Leaving this as a draft until it is resolved 😊

prost/prost-build/src/lib.rs

Lines 150 to 153 in eff0156

	// HELP(gibbz00): I think `error_message_protoc_not_found`, `protoc_from_env`, and `protoc_include_from_env`
	// should be made private. Only used inside config and not in any other part
	// of prost. Leaving this as is for now as this would technically be breaking
	// changes.

Depends on #1008.

[caspermeijn]

I already commented on the config split in #1020. I suggest doing the config and module split as two commits in a single PR.

[gibbz00]

Oki, I think I'll just close both and redo this in a new PR. Think it will be easier to resolve new conflicts that way.