Extract dag function out of v1alpha1 ⚙

The dag function shouldn't really be in v1alpha1 or even depend on
that package.

This removes `dag*.go` files from `v1alpha1` and move them to
`pkg/reconciler/pipeline/dag` package. This also removes dependency
from those function to `v1alpha1`, making it abstract and usable with
any other type that satisfy the `dag` package interface. This also
renames the `DAG` struct into `Graph` 👼

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>