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

lib/spack/llnl/util/filesystem.py

@@ -455,8 +455,10 @@ def working_dir(dirname, **kwargs):
 
     orig_dir = os.getcwd()
     os.chdir(dirname)
-    yield
-    os.chdir(orig_dir)
+    try:
+        yield
+    finally:
+        os.chdir(orig_dir)
 
 
 @contextmanager
@@ -605,6 +607,36 @@ def ancestor(dir, n=1):
     return parent
 
 
+def get_single_file(directory):
+    fnames = os.listdir(directory)
+    if len(fnames) != 1:
+        raise ValueError("Expected exactly 1 file, got {0}"
+                         .format(str(len(fnames))))
+    return fnames[0]
+
+
+@contextmanager
+def temp_cwd():
+    tmp_dir = tempfile.mkdtemp()
+    try:
+        with working_dir(tmp_dir):
+            yield tmp_dir
+    finally:
+        shutil.rmtree(tmp_dir)
+
+
+@contextmanager
+def temp_rename(orig_path, temp_path):
+    same_path = os.path.realpath(orig_path) == os.path.realpath(temp_path)
+    if not same_path:
+        shutil.move(orig_path, temp_path)
+    try:
+        yield
+    finally:
+        if not same_path:
+            shutil.move(temp_path, orig_path)
+
+
 def can_access(file_name):
     """True if we have read/write access to the file."""
     return os.access(file_name, os.R_OK | os.W_OK)