#2973: we should virtualise the CWD inside the GHC API, not in the client

#2973: we should virtualise the CWD inside the GHC API, not in the client
The problem is that we install the client's CWD before calling
runStmt, but runStmt has to load modules before running the code.  We
need to install the CWD just before running the code instead, which
means it has to be done inside runStmt (and resume).