This fixes up commit d5199b9d
("Add support for Python 3").

'environment-section' option is dropped.

It was broken because it conflicts with buildout,
which has similar feature.

In particular, buildout already strips values.

We want to keep the build directory when keep-compile-dir=true,
whereas sharing a part is reliable only if everything goes inside
the same directory.

1dc49d3e (tests: ignore CryptographyDeprecationWarning on python2, 2021-08-13)
was wrong, because this warning includes the path of the software, which
also has a md5sum hash, so the normalization has to be done before the
md5sum normalization, otherwise this consumes an md5sum and they become
off by one, with errors like:

    Expected:
        package: shared at /shared/package/<MD5SUM:0>
        Uninstalling package.
        Installing package.
        package: Checking whether package is installed at shared path: /shared/package/<MD5SUM:0>
        package: [ENV] FOO = bar
        package: Command 'set -e;./configure --prefix=/shared/package/<MD5SUM:0>"' returned non-zero exit status 127.
        package: Compilation error. The package is left as is at /shared/package/<MD5SUM:0>__compile__ where you can inspect what went wrong.
        A shell script slapos.recipe.build.env.sh has been generated. You can source it in your shell to reproduce build environment.
        /bin/sh: 1: ./configure: not found
        While:
          Installing package.
        Error: System error
    Got:
        package: shared at /shared/package/<MD5SUM:1>
        Uninstalling package.
        Installing package.
        package: Checking whether package is installed at shared path: /shared/package/<MD5SUM:1>
        package: [ENV] FOO = bar
        package: Command 'set -e;./configure --prefix=/shared/package/<MD5SUM:1>"' returned non-zero exit status 127.
        package: Compilation error. The package is left as is at /shared/package/<MD5SUM:1>__compile__ where you can inspect what went wrong.
        A shell script slapos.recipe.build.env.sh has been generated. You can source it in your shell to reproduce build environment.
        <BLANKLINE>
        /bin/sh: 1: ./configure: not found
        While:
          Installing package.
        Error: System error
        <BLANKLINE>

When we switch to using `python -m unittest discover -v` test_suite is not
used, but load_tests is used instead.

Symlinks can point anywhere outside the parts. Also symlinks can be
broken.

By using lstat instead of stat, the symlink is not considered as a
regular file.

See merge request !14

Don't force strip_top_level_dir option to be false when calling for slapos.recipe.build:downloadunpacked - let the user decide.

Effectively adds strip_top_level_dir option to the script which allows for shorter and more consistent paths.
Also it can help mitigating the variability of path length between archives downloaded for branch, commit hash and release.

Add some doctests to describe the use cases of shared parts with slapos,
including the need for our `${:_profile_base_location_}` buildout patch
and the need to save signature files.

It is redundant with the fact that slapos.buildout already rebuilds
if any dependency is modified. slapos.buildout does not yet understand
the new => syntax from upstream buildout but meanwhile a dummy option
can be set to any options of required parts.

The value was always '${:_profile_base_location_}'

In conjunction with the new Options.barrier() in buildout,
parts will be able to modify the environment during install/update.