Improve splitting lists of copyright holders

With:
```
(c) 1999 Terrehon Bowden
Bodo Bauer
```
we get
```
copyrights:
- copyright: (c) 1999 Terrehon Bowden Bodo Bauer
holders:
- holder: Terrehon Bowden Bodo Bauer
```
ideally we should get this instead
```
holders:
- holder: Terrehon Bowden
- holder: Bodo Bauer
```
The parse tree looks like this, so we have distinct name groups to guide splitting:
```
(label='ROOT', children=(
(label='COPYRIGHT', children=(
(label='COPY', value='(c)')
(label='NAME-YEAR', children=(
(label='NAME-YEAR', children=(
(label='NAME-YEAR', children=(
(label='YR-RANGE', children=(
(label='YR-RANGE', children=(
(label='YR', value='1999')
))
))
(label='NNP', value='Terrehon')
(label='NNP', value='Bowden')
))
))
))
(label='NAME-EMAIL', children=(
(label='NAME', children=(
(label='EMAIL', value='')
(label='NAME', children=(
(label='NNP', value='Bodo')
(label='NNP', value='Bauer')
))
))
(label='EMAIL', value='')
))
))
))
```