[OpenMP][NFC] Flatten plugin-nextgen/common folder sturcture (#73725)

For historic reasons we had it setup that there was
`  plugin-nextgen/common/PluginInterface/<sources + headers>`
which is not what we do anywhere else.
Now it looks like the rest:
```
  plugin-nextgen/common/include/<headers>
  plugin-nextgen/common/src/<sources>
```
As part of this, `dlwrap.h` was moved into common/include (as
`DLWrap.h`)
since it is exclusively used by the plugins.