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.
6ee2eb21dd