This PR gives users finer control over which specs are reused during concretization.
The value of the `concretizer:reuse` config option now can take an object with the following properties:
- `roots`: true if reusing roots, false if reusing just dependencies
- `exclude`: list of constraints used to select reusable specs
- `include`: list of constraints used to select reusable specs
- `from`: allows to select the sources of reused specs
### Examples
#### Reuse only specs compiled with GCC
```yaml
concretizer:
reuse:
roots: true
include:
- "%gcc"
```
#### `openmpi` must be used from externals, and it must be the only external used
```yaml
concretizer:
reuse:
roots: true
from:
- type: local
exclude:
- "openmpi"
- type: buildcache
exclude:
- "openmpi"
- type: external
include:
- "openmpi"
```
4e876b4014