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/bcl2fastq2/package.py

@@ -24,7 +24,6 @@
 ##############################################################################
 from spack import *
 import os
-import shutil
 import glob
 import llnl.util.tty as tty
 
@@ -94,7 +93,7 @@ def wrap():
                     tty.msg("cwd sez: {0}".format(os.getcwd()))
                     tarball = glob.glob(join_path('spack-expanded-archive',
                                         'bcl2fastq2*.tar.gz'))[0]
-                    shutil.move(tarball, '.')
+                    copy(tarball, '.')
                     os.rmdir('spack-expanded-archive')
                     tar = which('tar')
                     tarball = os.path.basename(tarball)