build env: simplify handling of parallel jobs (#11524)

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`)

var/spack/repos/builtin.mock/packages/a/package.py

@@ -32,6 +32,8 @@ class A(AutotoolsPackage):
 
     depends_on('b', when='foobar=bar')
 
+    parallel = False
+
     def with_or_without_fee(self, activated):
         if not activated:
             return '--no-fee'

var/spack/repos/builtin/packages/bazel/package.py

@@ -3,6 +3,8 @@
 #
 # SPDX-License-Identifier: (Apache-2.0 OR MIT)
 
+import inspect
+
 from spack import *
 from multiprocessing import cpu_count
 from spack.util.environment import env_flag
@@ -90,8 +92,9 @@ def __call__(self, *args, **kwargs):
                 return super(BazelExecutable, self).__call__(*args, **kwargs)
 
         jobs = cpu_count()
+        dependent_module = inspect.getmodule(dependent_spec.package)
         if not dependent_spec.package.parallel:
             jobs = 1
-        elif dependent_spec.package.make_jobs:
-            jobs = dependent_spec.package.make_jobs
+        elif dependent_module.make_jobs:
+            jobs = dependent_module.make_jobs
         module.bazel = BazelExecutable('bazel', 'build', jobs)