Fix refToPSet_ within VPSet

Fix (and add test for) ```refToPSet_``` expansion inside VPSet, e.g. in the following situation
```
process.a = cms.PSet(...)
process.b = cms.PSet(...)
process.c = cms.VPSet(
    cms.PSet(refToPSet_ = cms.string("a")),
    cms.PSet(refToPSet_ = cms.string("b"))
)
```