Having package-name flag in non-package interfaces causes them to be built as if
belonging to a package, which causes an issue for a loading client outside of the
package as follows.
For example, when building X that depends on A with the following dependency chain:
X --> A --> B --(package-only)--> C
1. X itself is not in the same package as A, B, and C.
2. When dependency scanning X, and opening up B, because the scan target is in a
different package domain, the scanner decides that B's package-only dependency
on C is to be ignored.
3. When then finally building A itself, it will load its dependencies, but because
the .private.swiftinterface of A still specifies -package-name, when it loads
B, it will then examine its dependencies and deem that this package-only dependency
on C is required.
Because (2) and (3) disagree, we get an error now when building the private A textual interface.
rdar://130701866
