* 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
- This moves var/spack/packages to var/spack/repos/builtin/packages.
- Packages that did not exist in the source branch, or were changed in
develop, were moved into var/spack/repos/builtin/packages as part of
the integration.
Conflicts:
lib/spack/spack/test/unit_install.py
var/spack/repos/builtin/packages/clang/package.py
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).
