1.5.0 causes `libFoundation` to be included in the binary

[sebsto]

A recent change introduced in version 1.5.0 (3acdb1c) causes libFoundation to be included in the binary.

In the Swift runtime for AWS Lambda project, we check for this as we have to produce the smallest possible binary and the change breaks our CI.

This change should be reverted and the lib must import FoundationEssentials instead of Foundation on non Darwin platforms

[sebsto]

@guoye-zhang FYI