internal/cuetxtar: error on txtar files with duplicate file entries

These could go unnoticed for a while as txtar allows them
and our code happily let the last duplicated file entry win.

There were two such duplicates already, both in cycle/issue990.txtar.

Signed-off-by: Daniel Martí <[email protected]>
Change-Id: Idb29d272534a09e455c01291a4bb20ff7df51c93
Reviewed-on: https://review.gerrithub.io/c/cue-lang/cue/+/1200855
TryBot-Result: CUEcueckoo <[email protected]>
Reviewed-by: Roger Peppe <[email protected]>

Author: mvdan

Diff for: cue/testdata/cycle/issue990.txtar

@@ -82,18 +82,6 @@ out.subs.subs.#p: invalid structural cycle
 This seems to be the result of incorrect handling of disjunctions.
 -- diff/todo/p3 --
 Reordering
--- diff/todo/p3 --
-Reordering
--- out/evalalpha/stats --
-Leaks:  2604
-Freed:  263
-Reused: 263
-Allocs: 2604
-Retain: 0
-
-Unifications: 239
-Conjuncts:    8656
-Disjuncts:    522
 -- out/evalalpha/stats --
 Leaks:  2075
 Freed:  218

Diff for: internal/cuetxtar/txtar.go

@@ -483,6 +483,9 @@ func (x *TxTarTest) run(t *testing.T, m *cuetdtest.M, f func(tc *Test)) {
 			// Track the position of the fallback files.
 			index := make(map[string]int, len(a.Files))
 			for i, f := range a.Files {
+				if _, ok := index[f.Name]; ok {
+					t.Errorf("duplicated txtar file entry %s", f.Name)
+				}
 				index[f.Name] = i
 			}