lib: avoid duplicate inclusion

For lib/log and lib/utilities, add double-inclusion protection as we're so early in startup that these are loaded explicitly rather than through `reloader.sh` or even the early lib load loop.
This *slightly* improves performance, but alsö improves debugging by reducing surface area.

lib/log.bash

@@ -1,4 +1,13 @@
-#!/usr/bin/env bash
+# shellcheck shell=bash
+#
+# A collection of logging functions.
+
+# Avoid duplicate inclusion
+if [[ -n "${__bash_it_lib_log:-}" ]]
+then
+    return 0
+fi
+__bash_it_lib_log="loaded"
 
 export BASH_IT_LOG_LEVEL_ERROR=1
 export BASH_IT_LOG_LEVEL_WARNING=2

lib/utilities.bash

@@ -2,6 +2,13 @@
 #
 # A collection of reusable functions.
 
+# Avoid duplicate inclusion
+if [[ -n "${__bash_it_lib_utilities:-}" ]]
+then
+    return 0
+fi
+__bash_it_lib_utilities="loaded"
+
 ###########################################################################
 # Generic utilies
 ###########################################################################