Fix Catch2 Single Header (#10022)

Fix the install of the single header amalgate in catch2.

The public API for catch < 2.3.0 was "#include <catch.hpp>".
For 2.3.0+ it is "#include <catch2/catch.hpp>"

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

@@ -63,6 +63,9 @@ def build(self, spec, prefix):
     @when('+single_header')
     def install(self, spec, prefix):
         mkdirp(prefix.include)
+        if spec.satisfies('@2.3.0:'):
+            install_tree('single_include', prefix.include)
+        else:
             install(join_path('single_include', 'catch.hpp'), prefix.include)
         # fakes out spack so it installs a module file
         mkdirp(join_path(prefix, 'bin'))