install_tree, copy_tree can install into existing directory structures (#8289)

Replace use of `shutil.copytree` with `copy_tree` and `install_tree` functions in `llnl.util.filesystem`.

- `copy_tree` copies without setting permissions.  It should be used to copy files around in the build directory.
- `install_tree` copies files and sets permissions.  It should be used to copy files into the installation directory.
- `install` and `copy` are analogous single-file functions.
- add more extensive tests for these functions
- update packages to use these functions.

var/spack/repos/builtin/packages/mpix-launch-swift/package.py

@@ -23,10 +23,9 @@
 # Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
 ##############################################################################
 from spack import *
-from distutils.dir_util import copy_tree
 
 
-class MpixLaunchSwift(Package):
+class MpixLaunchSwift(MakefilePackage):
     """Library that allows a child MPI application to be launched
     inside a subset of processes in a parent MPI application.
     """
@@ -42,5 +41,4 @@ class MpixLaunchSwift(Package):
     depends_on('swig', type='build')
 
     def install(self, spec, prefix):
-        make()
-        copy_tree('.', prefix)
+        install_tree('.', prefix)