The current `Spec.splice` model is very limited by the inability to splice specs that
contain multiple nodes with the same name. This is an artifact of the original
algorithm design predating the separate concretization of build dependencies,
which was the first feature to allow multiple specs in a DAG to share a name.
This PR provides a complete reimplementation of `Spec.splice` to avoid that
limitation. At the same time, the new algorithm ensures that build dependencies
for spliced specs are not changed, since the splice by definition cannot change
the build-time information of the spec. This is handled by splitting the dependency
edges and link/run edges into separate dependencies as needed.
Signed-off-by: Gregory Becker <becker33@llnl.gov>
07e964c688