ext4 was chosen for the 'discard' option, but formatting takes too
much time and space. I was surprised to see my VM being paused
(no left space on host) because of ext4lazyinit.

An unrelated change is about develop mode: the behaviour was simplified to do nothing if a working copy already exists.

TODO: handle removal of submodules

/reviewed-on !7

Note that we don't change the URL for BAD_GIT_REPOSITORY as we need them
to return 500 while trying to clone. If we use Nexedi Gitlab repo for
non-existing repo, we get redirected to login page. Hence, we keep using
git.erp5.org for `repo/nowhere`.

/reviewed-on !6

download, downloadunpacked: Check shared directory is not enough. Shared directory may be created before completing installation. Check signature file too.

Revert "download, downloadunpacked: Check shared directory is not enough. Shared directory may be created before completing installation. Check signature file too."

This reverts commit 9be7928b.

download, downloadunpacked: Check shared directory is not enough. Shared directory may be created before completing installation. Check signature file too.

downloadunpacked: Because shared part should not be uninstalled, install method should return an empty list in shared mode.

download, downloadunpacked: Rename 'shared-part' to 'shared-parts'. Since the original option name is 'parts', 'shared-parts' is consistent.

download, downloadunpacked: Disable shared feature by default since it is stil an experimental one. Enable it only if shared-part is defined.

This recipe alters a system perl ( or perl installed by another buildout
recipe ), which is not consistent with the independence of parts we are
trying to achieve in order to be able to cache and share parts.

SlapOS comonents now expose a `perl-CPAN-package` macro that should be
used for consistency. See slapos!341

/reviewed-on !4

For Stretch, the installer needs more than 256M.

setuptools.archive_util is not available unless setuptools.archive_util
have been imported before.

For example:
```python
>>> import setuptools
>>> setuptools.archive_util
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'archive_util'
>>> import setuptools.archive_util
>>> setuptools.archive_util
<module 'setuptools.archive_util' from '/usr/local/lib/python2.7/dist-packages/setuptools/archive_util.pyc'>
```

/reviewed-on !3

gitclone: Fix upload_network_cached. helper_upload_network_cached_from_directory needs two additional parameters since slapos.libnetworkcache 0.14.5.

Since slapos.libnetworkcache needs configuration, the user already
pulls it explicitely. Do not annoy those who don't use it.

In our case, it breaks offline builds with Python 2.6 when the
download-cache is prepared with Python 2.7: argparse missing.

zc.buildout.download.Download may return temporary files
(only readable/writable by the current user) and by default
we'd like installed files to respect umask.

In the case of SlapOS, we usually want installed files to be readable
by members of 'slapsoft' group.

Not fully tested. With this commit, it is at least possible to
bootstrap SlapOS from Python 3.