Add spack buildcache push (alias to buildcache create) (#34861)

`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`.

lib/spack/docs/binary_caches.rst

@@ -36,7 +36,7 @@ Build caches are created via:
 
 .. code-block:: console
 
-    $ spack buildcache create <path/url/mirror name> <spec>
+    $ spack buildcache push <path/url/mirror name> <spec>
 
 This command takes the locally installed spec and its dependencies, and
 creates tarballs of their install prefixes. It also generates metadata files,
@@ -48,7 +48,7 @@ Here is an example where a build cache is created in a local directory named
 
 .. code-block:: console
 
-    $ spack buildcache create --allow-root ./spack-cache ninja
+    $ spack buildcache push --allow-root ./spack-cache ninja
     ==> Pushing binary packages to file:///home/spackuser/spack/spack-cache/build_cache
 
 Not that ``ninja`` must be installed locally for this to work.
@@ -177,7 +177,7 @@ need to be adjusted for better re-locatability.
 --------------------
 
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
-``spack buildcache create``
+``spack buildcache push``
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 Create tarball of installed Spack package and all dependencies.