Adding `JExtractSwiftPlugin` links Full Foundation

[madsodgaard]

When adding the JExtractSwiftPlugin to a package, it has the side-effect of making the binary link against full Foundation:

U static Foundation.Bundle.main.getter : Foundation.Bundle
U Foundation.Bundle.__allocating_init(path: Swift.String) -> Foundation.Bundle?
U type metadata accessor for Foundation.Bundle

I am guessing this is due to some SPM plugin file copying or something?