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/farmhash/package.py

@@ -24,7 +24,6 @@
 ##############################################################################
 from spack import *
 import os.path
-from shutil import copyfile
 
 
 class Farmhash(CMakePackage):
@@ -36,5 +35,5 @@ class Farmhash(CMakePackage):
     version('92e897', commit='92e897b282426729f4724d91a637596c7e2fe28f')
 
     def patch(self):
-        copyfile(join_path(os.path.dirname(__file__), "CMakeLists.txt"),
-                 "CMakeLists.txt")
+        copy(join_path(os.path.dirname(__file__), "CMakeLists.txt"),
+             "CMakeLists.txt")