feat: add --from flag, refactor source providers (#2610)

Signed-off-by: Keith Zantow <kzantow@gmail.com>

syft/source/internal/digest_utils.go

@@ -0,0 +1,11 @@
+package internal
+
+import (
+	"strings"
+
+	"github.com/anchore/syft/syft/artifact"
+)
+
+func ArtifactIDFromDigest(input string) artifact.ID {
+	return artifact.ID(strings.TrimPrefix(input, "sha256:"))
+}