Fix flatMap key path type inference for Package@swift-5.9.swift

[CAMOBAP]

It seems like #2684 not fully fixed compilation issues for Swift 5.9 (Xcode 15.4)

/Package@swift-5.9.swift:113:31: error: cannot convert value of type 'WritableKeyPath<_, _>' to expected argument type '([Product]) throws -> Product?'
    ] as [[Product]]).flatMap(\.self),
                              ^
/Package@swift-5.9.swift:113:31: error: cannot infer key path type from context; consider explicitly specifying a root type
    ] as [[Product]]).flatMap(\.self),
                              ^
                               <#Root#>
/Package@swift-5.9.swift:126:30: error: cannot convert value of type 'WritableKeyPath<_, _>' to expected argument type '([Target]) throws -> Target?'
    ] as [[Target]]).flatMap(\.self),
                             ^
/Package@swift-5.9.swift:126:30: error: cannot infer key path type from context; consider explicitly specifying a root type
    ] as [[Target]]).flatMap(\.self),
                             ^
                              <#Root#> in https://github.com/ReactiveX/RxSwift

Full CLI call

Invalid manifest (compiled with: [
  "/Applications/Xcode_15.4.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swiftc",
  "-vfsoverlay", "<TMP_DIR>/vfs.yaml",
  "-L", "/Applications/Xcode_15.4.app/Contents/SharedFrameworks/SwiftPM.framework/SharedSupport/ManifestAPI",
  "-lPackageDescription",
  "-Xlinker", "-rpath",
  "-Xlinker", "/Applications/Xcode_15.4.app/Contents/SharedFrameworks/SwiftPM.framework/SharedSupport/ManifestAPI",
  "-target", "arm64-apple-macos13.0",
  "-sdk", "/Applications/Xcode_15.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk",
  "-swift-version", "5",
  "-I", "/Applications/Xcode_15.4.app/Contents/SharedFrameworks/SwiftPM.framework/SharedSupport/ManifestAPI",
  "-sdk", "/Applications/Xcode_15.4.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX14.5.sdk",
  "-package-description-version", "5.9.0",
  "-Xfrontend", "-serialize-diagnostics-path",
  "-Xfrontend", "<USER_CACHE>/org.swift.swiftpm/manifests/ManifestLoading/rxswift.dia",
  "/Package@swift-5.9.swift",
  "-disallow-use-new-driver",
  "-o", "<TMP_DIR>/rxswift-manifest"
])

[CAMOBAP]

@mitschlagel just wondering if you observe the issues mentioned in top comment

@freak4pc may I ask you to have a look

Thanks in advance

[mitschlagel]

Will take a look in the morning, cheers!