It's an EasyBuild feature, see `eb --avail-easyconfig-templates` for a list of currently supported templates.
Actually, you can get rid of `versiondir` all together by using `version_major` too:

```python
sources = ['hmmer-%(version)s-linux-intel-x86_64.tar.gz']
source_urls = ['http://selab.janelia.org/software/hmmer%(version_major)s/%(version)s/']
```

This is cleaner, will keep working when we move away from supporting Python code in easyconfigs, and is more explicit (imho)...