feat(wanda): Add dependency graph with @name discovery

Add buildDepGraph() to parse wanda specs and resolve @name dependencies
by scanning the repo for *.wanda.yaml files. The dependency graph is
built in deterministic topological order using Kahn's algorithm with
lexicographic tie-breaking.

Key features:
- @name syntax in froms references local specs by name
- Automatic discovery by scanning repo from git root
- Parallel spec parsing for performance
- Cycle detection with helpful error messages
- Variable expansion with unexpanded var detection

Topic: wanda-deps

Signed-off-by: andrew <andrew@anyscale.com>

Author: andrew-anyscale

wanda/deps.go

@@ -172,7 +172,7 @@ func TestForge_withHints(t *testing.T) {
 		t.Fatalf("build: %v", err)
 	}
 
-	const tag = "cr.ray.io/rayproject/hello"
+	const tag = "cr.ray.io/rayproject/hello-hint"
 
 	ref, err := name.ParseReference(tag)
 	if err != nil {

wanda/deps_test.go

@@ -0,0 +1,2 @@
+FROM scratch
+COPY Dockerfile.dep-base /opt/

wanda/forge_test.go

@@ -0,0 +1,2 @@
+FROM dep-base
+COPY world.txt /opt/

wanda/testdata/Dockerfile.dep-base

@@ -0,0 +1,2 @@
+FROM dep-middle
+COPY Dockerfile.dep-top /opt/

wanda/testdata/Dockerfile.dep-middle

@@ -0,0 +1,2 @@
+name: dep-base
+dockerfile: Dockerfile.dep-base

wanda/testdata/Dockerfile.dep-top

@@ -0,0 +1,5 @@
+name: dep-middle
+froms: ["cr.ray.io/rayproject/dep-base"]
+dockerfile: Dockerfile.dep-middle
+srcs:
+  - world.txt

wanda/testdata/dep-base.wanda.yaml

@@ -0,0 +1,3 @@
+name: dep-top
+froms: ["cr.ray.io/rayproject/dep-middle"]
+dockerfile: Dockerfile.dep-top

wanda/testdata/dep-middle.wanda.yaml

@@ -1,4 +1,4 @@
-name: hello
+name: hello-hint
 dockerfile: Dockerfile.hello
 build_hint_args:
   - MESSAGE=hint message