Promote to public operation to compute PIF GUID for products

[pmattos]

Motivation:

Ensures external clients (e.g., Xcode) can reuse some PIF GUID operations implemented internally by SwiftPM.

Without this patch, clients were instead duplicating the exact same code externally.

Modifications:

For now, this is a quick fix to ensure that we reuse this single function from SwiftBuildSupport/PackagePIFBuilder+Helpers.swift

static func targetGUID(
    forProductName name: String, 
    withId id: String, 
    suffix: TargetSuffix? = nil
) -> GUID

Related to rdar://165063807.

[pmattos]

@swift-ci test

[bkhouri]

@swift-ci test windows