Detect missing cpuinfo package when running test-libs.sh and let the user know (#967)

Due to the fact that the package name is not the same as the module name
(`py-cpuinfo` vs `cpuinfo`), the error that results from trying to run
this script when `py-cpuinfo` is not installed can be confusing. Let's
help users & developers by testing for the package and pointing them in
the right direction.

Also, slightly improve the clarity of the usage message text in this
script.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: mhucka

dev_tools/test_libs.sh

@@ -16,18 +16,27 @@
 set -eo pipefail -o errtrace
 
 declare -r usage="Usage: ${0##*/} [-h | --help | help] [bazel options ...]
-Run the programs in tests/, and on Linux, also build the programs in apps/.
+
+Invokes Bazel to run the programs in tests/, and on Linux, also build the
+sample programs in apps/.
 
 If the first option on the command line is -h, --help, or help, this help text
 will be printed and the program will exit. Any other options on the command
-line are passed directly to Bazel."
+line are passed directly to Bazel.
+
+This script makes use of the Python package 'py-cpuinfo'."
 
 # Exit early if the user requested help.
 if [[ "$1" == "-h" || "$1" == "--help" || "$1" == "help" ]]; then
     echo "$usage"
     exit 0
 fi
 
+if ! python -m pip show -qq py-cpuinfo 2>/dev/null; then
+    echo "Error: missing 'py-cpuinfo'. Please install dev-requirements.txt." >&2
+    exit 1
+fi
+
 # Look for AVX and SSE in the processor's feature flags.
 declare features=""
 declare filters=""