Composites from `variant` always expanded despite of being overridden

```
import feature ;
feature.feature foo
: on off
: propagated composite ;

feature.compose on : FOO_ON ;
feature.compose off : FOO_OFF ;

variant bar : release : on ;

exe test : test.cpp : bar off ;

#or:
#exe test : test.cpp : off ;
#>b2 bar
```

Compiles with `-DFOO_OFF -DFOO_ON`

```
exe simple : simple.cpp : off ;
>b2 foo=on
```

Correctly compiles with `-DFOO_OFF` only