how to change the include dir synchronously?

```cpp
// before bcp (normally use)
#include <boost/foo/bar.hpp>

boost::foo::bar bar;
```

```cpp
// after bcp (ideally use)
#include <toast/foo/bar.hpp>

toast::foo::bar bar;
```

* but now i can only use `#include <boost/foo/bar.hpp>` with `toast::foo::bar bar` instead