concretizer: change single-letter variables to descriptive names
concretizer: change single-letter variables to descriptive names

The original implementation was difficult to read, as it only had
single-letter variable names.  This converts all of them to descriptive
names, e.g., P -> Package, V -> Virtual/Version/Variant, etc.

iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,LLNL/spack,iulian787/spack,LLNL/spack,LLNL/spack,iulian787/spack,iulian787/spack