Install libsharp header files to include/libsharp. (#16847)

* Install libsharp header files to include/libsharp.

* Fix flake8 issues.

* Simplify specification of install path.

var/spack/repos/builtin/packages/libsharp/package.py

@@ -40,5 +40,7 @@ def configure_args(self):
         return args
 
     def install(self, spec, prefix):
-        install_tree('auto/include', join_path(prefix, 'include'))
-        install_tree('auto/lib', join_path(prefix, 'lib'))
+        # Libsharp's only caller healpix include headers like 'libsharp/xxx.h'
+        # Install xxx.h to include/libsharp
+        install_tree('auto/include', prefix.include.libsharp)
+        install_tree('auto/lib', prefix.lib)