Compiler search uses a pool of workers (#10190)

- spack.compilers.find_compilers now uses a multiprocess.pool.ThreadPool to execute
  system commands for the detection of compiler versions.

- A few memoized functions have been introduced to avoid poking the filesystem multiple
  times for the same results.

- Performance is much improved, and Spack no longer fork-bombs the system when doing a `compiler find`

lib/spack/llnl/util/multiproc.py

@@ -8,25 +8,9 @@
 than multiprocessing.Pool.apply() can.  For example, apply() will fail
 to pickle functions if they're passed indirectly as parameters.
 """
-from multiprocessing import Process, Pipe, Semaphore, Value
+from multiprocessing import Semaphore, Value
 
-__all__ = ['spawn', 'parmap', 'Barrier']
-
-
-def spawn(f):
-    def fun(pipe, x):
-        pipe.send(f(x))
-        pipe.close()
-    return fun
-
-
-def parmap(f, elements):
-    pipe = [Pipe() for x in elements]
-    proc = [Process(target=spawn(f), args=(c, x))
-            for x, (p, c) in zip(elements, pipe)]
-    [p.start() for p in proc]
-    [p.join() for p in proc]
-    return [p.recv() for (p, c) in pipe]
+__all__ = ['Barrier']
 
 
 class Barrier: