Publish the CPP plugin

Testing Done:
`./build-support/bin/release.sh -n` and confirmed the CPP plugin appeared in the output.

Bugs closed: 3930

Reviewed at https://rbcommons.com/s/twitter/r/4282/

contrib/release_packages.sh

@@ -110,6 +110,17 @@ function pkg_findbugs_install_test() {
       --explain compile | grep "findbugs" &> /dev/null
 }
 
+PKG_CPP=(
+  "pantsbuild.pants.contrib.cpp"
+  "//contrib/cpp/src/python/pants/contrib/cpp:plugin"
+  "pkg_cpp_install_test"
+)
+function pkg_cpp_install_test() {
+  execute_packaged_pants_with_internal_backends \
+      --plugins="['pantsbuild.pants.contrib.cpp==$(local_version)']" \
+      --explain compile | grep "cpp" &> /dev/null
+}
+
 # Once individual (new) package is declared above, insert it into the array below)
 CONTRIB_PACKAGES=(
   PKG_ANDROID
@@ -120,4 +131,5 @@ CONTRIB_PACKAGES=(
   PKG_PYTHON_CHECKS
   PKG_SCALAJS
   PKG_FINDBUGS
+  PKG_CPP
 )