--find-package (cmake's legacy pkg-config-like mode) silently requires
-DNAME=, -DLANGUAGE=, -DCOMPILER_ID= and -DMODE= to be set, or cmake
aborts with a chain of 'argument not specified' errors (see
Modules/CMakeFindPackageMode.cmake). The completion script gave no
hint that these were needed.
Detect --find-package on the command line and, when present, offer
NAME/LANGUAGE/COMPILER_ID/MODE alongside the usual -D property names,
plus value completion for each (e.g. LANGUAGE -> C/CXX/Fortran,
MODE -> EXIST/COMPILE/LINK).
Fixes#593
