solver: simplify edge-related functions in the solver #5086

Job	Run time
govulncheck	1m 19s
prepare	4s
binaries (darwin/amd64)	6m 11s
binaries (darwin/arm64)	1m 10s
binaries (linux/amd64)	2m 44s
binaries (linux/arm/v7)	10m 21s
binaries (linux/arm64)	2m 11s
binaries (linux/s390x)	10m 7s
binaries (linux/ppc64le)	5m 51s
binaries (linux/riscv64)	10m 12s
binaries (windows/amd64)	5m 26s
binaries (windows/arm64)	5m 31s
test / prepare	5m 47s
test / run (containerd, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	37m 35s
test / run (containerd-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integr...	8m 41s
test / run (containerd-1.6, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	11m 9s
test / run (containerd-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./fronte...	8m 53s
test / run (oci, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	6m 9s
test / run (oci-rootless, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, integration)	10m 12s
test / run (oci-rootless-slirp4netns-detachnetns, ./client ./cmd/buildctl ./worker/containerd ./solver ....	10m 42s
test / run (oci-snapshotter-stargz, ./client ./cmd/buildctl ./worker/containerd ./solver ./frontend, int...	10m 47s
test / run (./..., 1, integration gateway)	11m 8s
test / run (./client, containerd, nydus, integration)	6m 14s
test / run (./client, oci, nydus, integration)	8m 53s
test / run (./..., nydus, 1, integration)	8m 53s
image	0s
release	0s
scout	0s
	3h 26m 10s

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