Resources: use expanded archive name by default (#11688)

For resources, it is desirable to use the expanded archive name of
the resource as the name of the directory when adding it to the root
staging area.

#11528 established 'spack-src' as the universal directory where
source files are placed, which also affected the behavior of
resources managed with Stages.

This adds a new property ('srcdir') to Stage to remember the name of
the expanded source directory, and uses this as the default name when
placing a resource directory in the root staging area.

This also:

* Ensures that downloaded sources are archived using the expanded
  archive name (otherwise Spack will not be able to determine the
  original directory name when using a cached archive).
* Updates working_dir context manager to guarantee restoration of
  original working directory when an exception occurs
* Adds a "temp_cwd" context manager which creates a temporary
  directory and sets it as the working directory

var/spack/repos/builtin/packages/intel-xed/package.py

@@ -41,7 +41,7 @@ class IntelXed(Package):
         version(vers, commit=xed_hash)
         resource(name='mbuild',
                  git='https://github.com/intelxed/mbuild.git',
-                 commit=mbuild_hash, placement='mbuild',
+                 commit=mbuild_hash,
                  when='@{0}'.format(vers))
 
     variant('debug', default=False, description='Enable debug symbols')

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

@@ -40,19 +40,16 @@ class Warpx(MakefilePackage):
     resource(name='amrex',
              git='https://github.com/AMReX-Codes/amrex.git',
              when='@master',
-             tag='master',
-             placement='amrex')
+             tag='master')
 
     resource(name='amrex',
              git='https://github.com/AMReX-Codes/amrex.git',
              when='@dev',
-             tag='development',
-             placement='amrex')
+             tag='development')
 
     resource(name='picsar',
              git='https://bitbucket.org/berkeleylab/picsar.git',
-             tag='master',
-             placement='picsar')
+             tag='master')
 
     @property
     def build_targets(self):