internal/core/dep: first stab at precise dependency analyzer

Change-Id: I0bf5de9a1b2421ddffa4108c6dbffd38dae2cfe0
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/7604
Reviewed-by: CUE cueckoo <cueckoo@gmail.com>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/internal/core/dep/testdata/self.txtar b/internal/core/dep/testdata/self.txtar
new file mode 100644
index 0000000..70bf1f5
--- /dev/null
+++ b/internal/core/dep/testdata/self.txtar
@@ -0,0 +1,18 @@
+-- in.cue --
+a: b: {
+    for x in b {}
+
+    x: {
+        c: m
+        d: y
+        e: f
+        f: 1
+        g: b.x
+    }
+    y: 3
+}
+
+m: 3
+-- out/dependencies/field --
+-- out/dependencies/all --
+m