Ports: Only remove dir contents on clean, not the dir itself

The entirety of `.port_include.sh` depends on having a current working
directory for the respective port. If we were to remove the directory,
some actions such as `fetch` could fail since our current working
directory would now be an invalid inode.

This issue was exposed by running `./package.sh` followed by
`./package.sh dev` and answering 'y' to the question on cleaning the
build directory.

Ports/.port_include.sh

@@ -480,7 +480,7 @@ func_defined post_install || post_install() {
     :
 }
 clean() {
-    rm -rf "${PORT_BUILD_DIR}"
+    rm -rf "${PORT_BUILD_DIR}/"*
 }
 clean_dist() {
     OLDIFS=$IFS