* cmd/search: Add support for search of the packages
with a specific label.
* review Search method of Repositories for permit
different search modes.
* labels are k/v attributes and could be matched
through label key (with HasLabel method) or through
regex that use "$key" + "=" + "$value"
Add "provides" field in packages (which affect both runtime and buildtime deps).
It replaces all the occurences in the deptree before solving, actually
allowing to swap packages and provide virtuals. Along with a mechanism
for package rename #25.
The version inmemory is optimized, while the boltdb implementation is
not.
It returns a list of the same package but with all the versions present
in the db.
Unmarshalling needs well-defined structs, also swap pointers in few places where needed.
There are few more TODOS pending (like getting UpdatePackage fixed properly)
