Add the ability to the modules generation process to blacklist
packages that were installed implicitly. One can still whitelist
modules that were installed implicitly.
This changes adds a `blacklist_implicts` boolean as a peer to the
`whitelist` and `blacklist` arrays, e.g.:
```
modules:
enable::
- lmod
lmod:
whitelist:
- 'lua'
- 'py-setuptools'
blacklist:
- '%gcc@4.8.3'
blacklist_implicits: True
```
It adds a small helper in `spec.py` and then touches up the package
filtering code in `modules.py`.
5a836fd06e