This changes the hash algorithm so that it does much less object
allocation and copying, and so that it is correct.
The old version of `_cmp_key()` would call `sorted_deps`, which would
call `flat_dependencies` to get a list of dependencies so that it
could sort them in alphabetical order. This isn't necessary in the
`_cmp_key()`, and in fact we want more DAG structure than that to be
included in the `_cmp_key()`.
The new version constructs a tuple without copying the Spec DAG, and
the tuple contains hashes of sub-DAGs that are computed recursively
in-place. This is way faster than the previous algorithm and reduces
the numebr of copies significantly. It is also a correct DAG hash.
Example timing and copy counts for the different hashing algorithms
we've tried:
Original (wrong) Spec hash:
```
106,170 copies
real 0m5.024s
user 0m4.949s
sys 0m0.104s
```
Spec hash using YAML `dag_hash()`:
```
3,794 copies
real 0m5.024s
user 0m4.949s
sys 0m0.104s
New no-copy, no-YAML hash:
```
3,594 copies
real 0m2.543s
user 0m2.435s
sys 0m0.104s
```
So now we have a hash that is correct AND faster.
The remaining ~3k copies happen mostly during concretization, and as
all packages are initially loaded. I believe this is because Spack
currently has to load all packages to figure out virtual dependency
information; it could also be becasue there ar a lot of lookups of
partial specs in concretize. I can investigate this further.
Flux package reworked to include all new dependencies, fixed issues with a
clean-build of hwloc, lua and czmq as well that prevented flux from building
cold on a minimal system.
- _cross_provider_maps() had suffered some bit rot (map returned was
ill-formed but still worked for cases with one vdep)
- ProviderIndex.satisfies() was only checking whether the result map
was non-empty. It should check whether all common vdeps are *in*
the result map, as that indicates there is *some* way to satisfy
*all* of them. We were checking whether there was some way to
satisfy *any one* of them, which is wrong.
- Above would cause a problem when there is more than one vdep provider.
- Added test that covers this case.
- Added `constrained()` method to Spec. Analogous to `normalized()`:
`constrain():constrained() :: normalize():normalized()`
