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/scr/package.py

@@ -25,7 +25,6 @@
 from spack import *
 
 import os
-import shutil
 
 
 class Scr(CMakePackage):
@@ -151,4 +150,4 @@ def copy_config(self):
         if spec.variants['copy_config'].value:
             dest_path = self.get_abs_path_rel_prefix(
                 spec.variants['scr_config'].value)
-            shutil.copyfile(spec.variants['copy_config'].value, dest_path)
+            install(spec.variants['copy_config'].value, dest_path)