pkg2: ported resolve code
pkg2/resolve: remove vers (use deps instead)
pkg2/resolve: improve vdict
pkg2: fix VersionSet equality operator

also add hashing
pkg2/resolve: make FieldValue immutable

also:
 * move FieldValue to its own file
 * fix sanity_check error object construction
 * back to BitMatrix in dependencies masks
 * cosmetics
pkg2/query: small fix in requirements
pkg2/resolve: split code in separate files

also fix a typo in impossible requirements reporting
pkg2/resolve: merge Reqs/PkgStruct into Interface

also renamed things accordingly
pkg2: added check_metadata

used to be check_repository in Pkg