Since python 3.3, collections has been moved to collections.abc module.
For backwards compatibility, they continue to be visible in this module through Python 3.7.
Subsequently, they will be removed entirely.

See https://docs.python.org/3/library/collections.html

Fix python warnings: Use collections.abc instead collections

See merge request !883

We shouldn't need it to persist now that we cache failed build dirs.

This change breaks the test
`tests/integration/shell.py::test_sysroot_workspace_visible`.
I can no longer see a use-case for this test.
AIUI, it tested that the failed build sysroot stored in the builddir has
the workspace's files in, despite the workspace being unmounted.
I believe this behaviour is made redundant by cached buildtrees.

This fixes part of #539

This includes changes in app.py:
* Interactive failure shell no longer uses the failed build sysroot,
  defaulting to the cached build tree.
* Always allow producing a failure shell.

Changes in element.py are:
* Errors caused by building don't store the failed build sysroot.
* When staging sources, will stage the element's cached build tree if it
  exists.

This fixes part of #539

This is related to #539

This test checks that the interactive failure shell stages the cached
build tree.

i.e. instead of taking a directory that must contain "root" and
"scratch", and treating "root" as the root, use the directory directly.

In element.py:
* __sandbox takes the `bare_sandbox` arg, to pass into the sandbox's constructor

In sandbox.py:
* If bare_sandbox, `_root` is the passed-in directory, and `__scratch`
  is None.
* Trying to use `__scratch` when bare_sandbox is True is a bug.

In _mount.py:
* Don't get the value of `__scratch` if it's not needed.

This is part of #539

This is related to #539