This PR implements several refactors requested in #11373, specifically:
- Config scopes are used to handle builtin defaults, command line overrides
and package overrides (`parallel=False`)
- `Package.make_jobs` attribute has been removed; `make_jobs` remains
as a module-scope variable in the build environment.
- The use of the argument `-j` has been rationalized across commands
- move '-j'/'--jobs' argument into `spack.cmd.common.arguments`
- Add unit tests to check that setting parallel jobs works as expected
- add new test to ensure that build job setting is isolated to each build
- Fix packages that used `Package.make_jobs` (i.e. `bazel`)
c291866b9a