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/alias.txtar b/internal/core/dep/testdata/alias.txtar
new file mode 100644
index 0000000..51f3efa
--- /dev/null
+++ b/internal/core/dep/testdata/alias.txtar
@@ -0,0 +1,14 @@
+-- in.cue --
+let X = Y
+let Y = c + d
+
+a: b: X
+
+c: 5
+d: 6
+-- out/dependencies/field --
+c
+d
+-- out/dependencies/all --
+c
+d