[hxb] JVM roundtrip doesn't work anymore

```
PS C:\git\haxe\tests\unit> haxe .\compile-hxb-jvm-roundtrip.hxml
Fatal error: exception HxbData.HxbFailure("Could not read static field printNow on unit.TestMainNow while hxbing unit.TestMain")
Raised at HxbReader.hxb_reader#read_cfr.(fun) in file "src/compiler/hxb/hxbReader.ml", line 1703, characters 7-162
```

This used to work at some point. The idea is that `TestMainNow`, which is `exclude`d in hxb-config/jvm.json, is loaded normally, while the rest is retrieved from the HXB lib. I already checked that the class isn't in the archive, so the problem must be related to the reader somehow.