Do not validate unused SCM dependencies

[neonichu]

Currently, we are doing unconditional validation of any SCM dependencies which can lead to unexpected and confusing results, e.g. the branch of a SCM dependency that is overidden by a root failing validation and breaking builds. We should only validate what we actually use.

rdar://117442643

[neonichu]

I'm not sure how most of the loadRoot* APIs in workspace are supposed to be used and are supposed to work.

It seems like many of them are used interchangeably even though they seem mutually exclusive e.g. loadRootManifests() vs. loadRootPackage(). Because of this I opted for the client always providing the list of root package identities since that is the only one who can know them based on the current API surface. That said, the client mostly also doesn't have this information, so we can just pass in what we know about at the time...

[tomerd]

let see if there are alternative approaches

[neonichu]

let see if there are alternative approaches

I think we were saying we would consider dropping the SCM validation at this stage.

Separately, we also need to change when these validations happen since currently they're performed multiple times in a single SwiftPM invocation, leading to duplicated diagnostics.

[neonichu]

Closing in favor of #7048