Adds SpackCommand class allowing Spack commands to be easily in Python
Example usage:
from spack.main import SpackCommand
info = SpackCommand('info')
out, err = info('mpich')
print(info.returncode)
This allows easier testing of Spack commands.
Also:
* Simplify command tests
* Simplify mocking in command tests.
* Simplify module command test
* Simplify python command test
* Simplify uninstall command test
* Simplify url command test
* SpackCommand uses more compatible output redirection
* SV variants are evaluated correctly in `when=` statements fixes#4113
The problem here was tricky:
```python
spec.satisfies(other)
```
changes already the MV variants in others into SV variants (where
necessary) if spec is concrete. If it is not concrete it does
nothing because we may be acting at a pure syntactical level.
When evaluating a `when=` keyword spec is for sure not concrete
as it is in the middle of the concretization process. In this case we
have to trigger manually the substitution in other to not end up
comparing a MV variant "foo=bar" to a SV variant "foo=bar" and having
False in return. Which is wrong.
* sv variants: improved error message for typos in "when=" statements
Modifications:
- added support for multi-valued variants
- refactored code related to variants into variant.py
- added new generic features to AutotoolsPackage that leverage multi-valued variants
- modified openmpi to use new features
- added unit tests for the new semantics
Package repositories now look like this:
top-level-dir/
repo.yaml
packages/
libelf/
package.py
mpich/
package.py
...
This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).
