`spack buildcache create` is a misnomer cause it's the only way to push to
an existing buildcache (and it in fact calls binary_distribution.push).
Also we have `spack buildcache update-index` but for create the flag is
`--rebuild-index`, which is confusing (and also... why "rebuild"
something if the command is "create" in the first place, that implies it
wasn't there to begin with).
So, after this PR, you can use either
```
spack buildcache create --rebuild-index
```
or
```
spack buildcache push --update-index
```
Also, alias `spack buildcache rebuild-index` to `spack buildcache
update-index`.
9ef062fcca