Issue Description:

SwiftPM uses rsync here as part of its build process. If rsync is not found on the system (say, a minimal container without many dependencies) then it will fail with a terse/cryptic error message:

Compile Swift Module 'Commands' (14 sources)
Compile Swift Module 'swift_package' (1 sources)
Compile Swift Module 'swift_build' (1 sources)
Compile Swift Module 'swift_test' (1 sources)
Compile Swift Module 'swift_run' (1 sources)

• • • bootstrap: note: building runtime v4_2 target: PackageDescription4:
      Compile Swift Module 'PackageDescription' (9 sources)

    • bootstrap: note: building runtime v3 target: PackageDescription:
      Compile Swift Module 'PackageDescription' (6 sources)

    • bootstrap: note: building runtime v4 target: PackageDescription4:
      Compile Swift Module 'PackageDescription' (9 sources)
      Traceback (most recent call last):
      File "/swift-source/swiftpm/Utilities/bootstrap", line 1276, in <module>
      main()
      File "/swift-source/swiftpm/Utilities/bootstrap", line 949, in main
      build, args, lib_path)
      File "/swift-source/swiftpm/Utilities/bootstrap", line 659, in process_runtime_libraries
      subprocess.check_call(cmd)
      File "/usr/lib/python2.7/subprocess.py", line 185, in check_call
      retcode = call(*popenargs, **kwargs)
      File "/usr/lib/python2.7/subprocess.py", line 172, in call
      return Popen(*popenargs, **kwargs).wait()
      File "/usr/lib/python2.7/subprocess.py", line 394, in init
      errread, errwrite)
      File "/usr/lib/python2.7/subprocess.py", line 1047, in _execute_child
      raise child_exception
      OSError: [Errno 2] No such file or directory
      ./swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting
      ./swift/utils/build-script: fatal error: command terminated with a non-zero exit status 1, aborting

It would be better if a fallback position determined that rsync was not available, and used a different mechanism (and/or failed with a sensible error message) instead of the above