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/llvm/package.py

@@ -504,10 +504,9 @@ def check_darwin_lldb_codesign_requirement(self):
         if not self.spec.satisfies('+lldb platform=darwin'):
             return
         codesign = which('codesign')
-        cp = which('cp')
         mkdir('tmp')
         llvm_check_file = join_path('tmp', 'llvm_check')
-        cp('/usr/bin/false', llvm_check_file)
+        copy('/usr/bin/false', llvm_check_file)
 
         try:
             codesign('-f', '-s', 'lldb_codesign', '--dryrun',