Issue Description:

For the record, this comes from here: grpc/grpc-swift#887

It seems like `swift package generate-xcodeproj` sometimes wrongly guesses that a dependency requires iOS 13.5.

Here are detailed steps to reproduce with SwiftProtobuf:

1. Create a new project with `swift package init`.
2. In `dependencies`, add `.package(name: "SwiftProtobuf", url: "https://github.com/apple/swift-protobuf.git", from: "1.9.0"),
3. In `targets: (dependencies`, add `SwiftProtobuf`)
4. Run `swift package generate-xcodeproj`
5. Run `open <project-name>.xcodeproj` to open with Xcode
6. In the (only) source file "<project>.swift", add the line: `import SwiftProtobuf`
7. Try to build for "iPhone 11".

You should see the following error:

> Compiling for iOS 8.0, but module 'SwiftProtobuf' has a minimum deployment target of iOS 13.5: /path/to/x86_64-apple-ios-simulator.swiftmodule

But SwiftProtobuf does not require iOS 13.5.