Speed-up environment concretization on linux with a process pool (#26264)

* Speed-up environment concretization with a process pool

We can exploit the fact that the environment is concretized
separately and use a pool of processes to concretize it.

* Add module spack.util.parallel

Module includes `pool` and `parallel_map` abstractions,
along with implementation details for both.

* Add a new hash type to pass specs across processes

* Add tty msg with concretization time

var/spack/repos/builtin.mock/packages/flake8/package.py

@@ -39,7 +39,7 @@ class Flake8(Package):
     variant('super-awesome-feature',    default=True,  description='Enable super awesome feature')
     variant('somewhat-awesome-feature', default=False, description='Enable somewhat awesome feature')
 
-    provides('lapack', when='@2.0+super-awesome-feature+somewhat-awesome-feature')
+    provides('somevirt', when='@2.0+super-awesome-feature+somewhat-awesome-feature')
 
     extends('python', ignore='bin/(why|does|every|package|that|depends|on|numpy|need|to|copy|f2py3?)')