Refactor "spack buildcache" command (#27776)

This PR is meant to move code with "business logic" from `spack.cmd.buildcache` to appropriate core modules[^1]. 

Modifications:
- [x] Add `spack.binary_distribution.push` to create a binary package from a spec and push it to a mirror
- [x] Add `spack.binary_distribution.install_root_node` to install only the root node of a concrete spec from a buildcache (may check the sha256 sum if it is passed in as input)
- [x] Add `spack.binary_distribution.install_single_spec` to install a single concrete spec from a buildcache
- [x] Add `spack.binary_distribution.download_single_spec` to download a single concrete spec from a buildcache to a local destination
- [x] Add `Spec.from_specfile` that construct a spec given the path of a JSON or YAML spec file
- [x] Removed logic from `spack.cmd.buildcache`
- [x] Removed calls to `spack.cmd.buildcache` in `spack.bootstrap`
- [x] Deprecate `spack buildcache copy` with a message that says it will be removed in v0.19.0

[^1]: The rationale is that commands should be lightweight wrappers of the core API, since that helps with both testing and scripting (easier mocking and no need to invoke `SpackCommand`s in a script).

share/spack/spack-completion.bash

@@ -511,7 +511,7 @@ _spack_buildcache_create() {
 _spack_buildcache_install() {
     if $list_options
     then
-        SPACK_COMPREPLY="-h --help -f --force -m --multiple -a --allow-root -u --unsigned -o --otherarch --sha256 --only-root"
+        SPACK_COMPREPLY="-h --help -f --force -m --multiple -a --allow-root -u --unsigned -o --otherarch"
     else
         _all_packages
     fi