Issue Description:

For eg this in Package.swift resulted in an assertion below. We should diagnose and report a proper message.

Target(
name: "XcodeprojTests",
dependencies: ["XcodeprojTests", "TestSupport"]),

• • • bootstrap: note: building self-hosted 'swift-build': env SWIFT_EXEC=/Users/ankit/workspace/opensource/swift.org/swiftpm/.build/debug/swiftc SWIFT_BUILD_PATH=/Users/ankit/workspace/opensource/swift.org/swiftpm/.build SYSROOT=/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.12.sdk /Users/ankit/workspace/opensource/swift.org/swiftpm/.build/debug/swift-build-stage1 -Xlinker -rpath -Xlinker @executable_path/../lib/swift/macosx --build-tests
      fatal error: 'try!' expression unexpectedly raised an error: Basic.GraphError.unexpectedCycle: file /Library/Caches/com.apple.xbs/Sources/swiftlang/swiftlang-800.0.46.2/src/swift/stdlib/public/core/ErrorType.swift, line 178
      Current stack trace:
      0 libswiftCore.dylib 0x0000000105c37580 swift_reportError + 132
      1 libswiftCore.dylib 0x0000000105c54730 _swift_stdlib_reportFatalErrorInFile + 112
      2 libswiftCore.dylib 0x0000000105c03700 partial apply for (_assertionFailed(StaticString, String, StaticString, UInt, flags : UInt32) -> Never).(closure Add missing steps to creating a C library package. #1).(closure Add missing steps to creating a C library package. #1).(closure Add missing steps to creating a C library package. #1) + 99
      3 libswiftCore.dylib 0x0000000105a50550 specialized specialized StaticString.withUTF8Buffer<A> ((UnsafeBufferPointer<UInt8>) -> A) -> A + 355
      4 libswiftCore.dylib 0x0000000105c03640 partial apply for (_assertionFailed(StaticString, StaticString, StaticString, UInt, flags : UInt32) -> Never).(closure Add missing steps to creating a C library package. #1).(closure Add missing steps to creating a C library package. #1) + 144
      5 libswiftCore.dylib 0x0000000105a50a60 specialized specialized String._withUnsafeBufferPointerToUTF8<A> ((UnsafeBufferPointer<UInt8>) throws -> A) throws -> A + 124
      6 libswiftCore.dylib 0x0000000105ba9290 partial apply for (_assertionFailed(StaticString, String, StaticString, UInt, flags : UInt32) -> Never).(closure Add missing steps to creating a C library package. #1) + 185
      7 libswiftCore.dylib 0x0000000105a50550 specialized specialized StaticString.withUTF8Buffer<A> ((UnsafeBufferPointer<UInt8>) -> A) -> A + 355
      8 libswiftCore.dylib 0x0000000105a50330 _assertionFailed(StaticString, String, StaticString, UInt, flags : UInt32) -> Never + 144
      9 libswiftCore.dylib 0x0000000105a74000 swift_unexpectedError_merged + 585
      10 swift-build-stage1 0x000000010582e380 Module.recursiveDependencies.getter + 224
      11 swift-build-stage1 0x00000001057a00d0 Module.XccFlags(AbsolutePath) -> [String] + 43
      12 swift-build-stage1 0x00000001057a5d60 static Command.compile(swiftModule : SwiftModule, configuration : Configuration, prefix : AbsolutePath, otherArgs : [String], compilerExec : AbsolutePath) throws -> Command + 226
      13 swift-build-stage1 0x00000001057b5300 describe(AbsolutePath, Configuration, PackageGraph, flags : BuildFlags, toolchain : Toolchain) throws -> AbsolutePath + 2784
      14 swift-build-stage1 0x00000001056f3d30 SwiftBuildTool.run() -> () + 2335
      15 swift-build-stage1 0x00000001056eaf20 main + 114
      16 libdyld.dylib 0x00007fff9900f5ac start + 1

    • bootstrap: error: build failed with exit status -4