solver: simplify edge-related functions in the solver #4547

Job	Run time
prepare	1s
test / prepare	1m 50s
test / run (containerd, ./frontend/dockerfile, integration)	4m 26s
test / run (containerd, ./frontend/dockerfile, dockerfile)	7m 20s
test / run (containerd-rootless, ./frontend/dockerfile, integration)	4m 45s
test / run (containerd-rootless, ./frontend/dockerfile, dockerfile)	6m 59s
test / run (containerd-1.6, ./frontend/dockerfile, integration)	5m 20s
test / run (containerd-1.6, ./frontend/dockerfile, dockerfile)	6m 26s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, integration)	5m 1s
test / run (containerd-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	7m 0s
test / run (oci, ./frontend/dockerfile, integration)	4m 19s
test / run (oci, ./frontend/dockerfile, dockerfile)	5m 31s
test / run (oci-rootless, ./frontend/dockerfile, integration)	4m 42s
test / run (oci-rootless, ./frontend/dockerfile, dockerfile)	5m 56s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, integration)	4m 21s
test / run (oci-rootless-slirp4netns-detachnetns, ./frontend/dockerfile, dockerfile)	5m 28s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, integration)	4m 23s
test / run (oci-snapshotter-stargz, ./frontend/dockerfile, dockerfile)	6m 11s
image (mainline)	4m 53s
image (labs)	5m 2s
release	1s
scout	1s
	1h 39m 56s

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