Add option to install targets even when not top level

Also add namespaced targets for any exported target to facilitate
find_package and FetchContent both using the same target names.

Author: bnbarham

cmake/modules/CMakeLists.txt

@@ -1,5 +1,9 @@
-if(PROJECT_IS_TOP_LEVEL)
+if(PROJECT_IS_TOP_LEVEL OR SWIFT_SYNTAX_INSTALL_TARGETS)
   export(EXPORT SwiftSyntaxTargets
          FILE ${CMAKE_CURRENT_BINARY_DIR}/SwiftSyntaxConfig.cmake
          NAMESPACE SwiftSyntax::)
 endif()
+
+foreach(OrigTarget ${SwiftSyntaxTargets})
+  add_library(SwiftSyntax::${OrigTarget} ALIAS ${OrigTarget})
+endforeach()