Add `when` context manager to group common constraints in packages (#24650)

This PR adds a context manager that permit to group the common part of a `when=` argument and add that to the context:
```python
class Gcc(AutotoolsPackage):
    with when('+nvptx'):
        depends_on('cuda')
        conflicts('@:6', msg='NVPTX only supported in gcc 7 and above')
        conflicts('languages=ada')
        conflicts('languages=brig')
        conflicts('languages=go')
```
The above snippet is equivalent to:
```python
class Gcc(AutotoolsPackage):
    depends_on('cuda', when='+nvptx')
    conflicts('@:6', when='+nvptx', msg='NVPTX only supported in gcc 7 and above')
    conflicts('languages=ada', when='+nvptx')
    conflicts('languages=brig', when='+nvptx')
    conflicts('languages=go', when='+nvptx')
```
which needs a repetition of the `when='+nvptx'` argument. The context manager might help improving readability and permits to group together directives related to the same semantic aspect (e.g. all the directives needed to model the behavior of `gcc` when `+nvptx` is active). 

Modifications:

- [x] Added a `when` context manager to be used with package directives
- [x] Add unit tests and documentation for the new feature
- [x] Modified `cp2k` and `gcc` to show the use of the context manager 

0 files changed, 0 insertions, 0 deletions