txtar: promote from internal

Rather than copying this package to another repository, let's promote
this one out of internal.

Change-Id: I6f9cc1ada1577a720905271f7471c3afe05a2b41
Reviewed-on: https://go-review.googlesource.com/c/tools/+/207905
Reviewed-by: Ian Cottrell <iancottrell@google.com>

internal/imports/mod_test.go

@@ -19,7 +19,7 @@ import (
 	"golang.org/x/tools/internal/gopathwalk"
 	"golang.org/x/tools/internal/module"
 	"golang.org/x/tools/internal/testenv"
-	"golang.org/x/tools/internal/txtar"
+	"golang.org/x/tools/txtar"
 )
 
 // Tests that we can find packages in the stdlib.

internal/lsp/tests/tests.go

@@ -26,7 +26,7 @@ import (
 	"golang.org/x/tools/internal/lsp/protocol"
 	"golang.org/x/tools/internal/lsp/source"
 	"golang.org/x/tools/internal/span"
-	"golang.org/x/tools/internal/txtar"
+	"golang.org/x/tools/txtar"
 )
 
 const (