HPCC-2839 Command line tool to validate package content

Adds a WsPackageProcess web service method "ValidatePackage" and an
"ecl packagemap validate" command line option to validate a packagemap
file.

Checks some common definition errors, and (based on a specified
target cluster) will check for unused package definitions and unmatched
queries.

Signed-off-by: Anthony Fishbeck <Anthony.Fishbeck@lexisnexis.com>