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

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