Don't Validate if we have an execution plan

The plan file should contain all data required to execute the apply
operation. Validation requires interpolation, and the `file()`
interpolation function may fail if the module files are not present.
This is the case currently with how TFE executes plans.

command/meta_backend.go

@@ -104,9 +104,13 @@ func (m *Meta) Backend(opts *BackendOpts) (backend.Enhanced, error) {
 		StateBackupPath: m.backupPath,
 		ContextOpts:     m.contextOpts(),
 		Input:           m.Input(),
-		Validation:      true,
 	}
 
+	// Don't validate if we have a plan.  Validation is normally harmless here,
+	// but validation requires interpolation, and `file()` function calls may
+	// not have the original files in the current execution context.
+	cliOpts.Validation = opts.Plan == nil
+
 	// If the backend supports CLI initialization, do it.
 	if cli, ok := b.(backend.CLI); ok {
 		if err := cli.CLIInit(cliOpts); err != nil {