This shouldn't have any behavior changes for now. This is a code
refactor that should allow us to do further improvements on how
variables are handled, specially regarding respecting the declaration
order in Taskfiles, which should make it easier for the users.
Initial work on #218
Also, fix handling of Taskfile by making the version an instance of
`semver.Constraints` instead of `semver.Version`. This makes the version
works as described on TASKFILE_VERSIONS.md document, i.e. version "2" will
include "2.x" features but version "2.0" not.