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/gaussian/package.py

@@ -24,7 +24,6 @@
 ##############################################################################
 from spack import *
 import os
-import shutil
 
 
 class Gaussian(Package):
@@ -36,7 +35,7 @@ class Gaussian(Package):
     version('09', '7d4c95b535e68e48af183920df427e4e')
 
     def install(self, spec, prefix):
-        shutil.copytree(os.getcwd(), prefix.bin)
+        install_tree('.', prefix.bin)
         patch_install_files = ['flc',
                                'linda8.2/opteron-linux/bin/flc',
                                'linda8.2/opteron-linux/bin/LindaLauncher',