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/alglib/package.py

@@ -26,7 +26,6 @@
 import glob
 import os
 import sys
-import shutil
 
 
 class Alglib(MakefilePackage):
@@ -48,7 +47,7 @@ def edit(self, spec, prefix):
         make_file_src = join_path(os.path.dirname(self.module.__file__),
                                   'Makefile')
         make_file = join_path(self.stage.source_path, 'src', 'Makefile')
-        shutil.copy(make_file_src, make_file)
+        copy(make_file_src, make_file)
         filter_file(r'so', dso_suffix, make_file)
 
     def install(self, spec, prefix):