Make ament_cmake_python_install() handle symlink installs.

[hidmic]

Precisely what the title says. Yet another regression after #316, though this one turned into a rabbit hole. colcon handles Python packages in a very peculiar way, so this patch replicates as much as necessary into ament_cmake_python.

Full CI, just to be sure:

• Linux
• Linux-aarch64
• macOS
• Windows

[hidmic]

Running CI for Windows:

• Windows

[ivanpauno]

LGTM!
It would be great if someone else can also take a look too.

I will also test locally before ✔️

[ivanpauno]

I would recommend running CI again with --symlink-install, just to double check everything is fine

[hidmic]

Test failures in full CI are known and can be found in nightlies as well.

CI using --symlink-install as per request, but up to rclpy (should be enough to reveal any bugs):

• Linux
• Linux-aarch64
• macOS
• Windows

[ivanpauno]

CI using --symlink-install as per request, but up to rclpy (should be enough to reveal any bugs):

Just in case, it would be good to also test something that depends on rclpy.
While using this branch, I had problems with rclpy dependencies not being able to find the _rclpy shared library (I'm not sure way, maybe PYTHONPATH was wrong (?)).

[hidmic]

Ok, Windows CI is failing because some hard-coded relative path in rosidl_typesupport_connext_c is no longer valid for symlink installs. Unix CIs are trickier. I cannot reproduce locally, which makes me think something's special about their filesystem layout.

[hidmic]

Alright, it's worst than I thought. There's a lot of packages that rely on Python sources (or a symlink to them) being present in the install space... I'll drop the environment hooks and do plain symlinks. It's not more of a hack than using these ad-hoc hooks.

[ivanpauno]

LGTM if CI is happy

[hidmic]

CI using --symlink-install up to rclpy and test_communication:

• Linux
• Linux-aarch64
• macOS
• Windows

[hidmic]

Argh... this is so fragile.

[hidmic]

This patch is an abomination, but I don't know how to make things better.

• Linux
• Linux-aarch64
• macOS
• Windows

BTW there's also an uninstall target that this macro does not implement, but it doesn't seem to be working well either way. I have some prototype code implementing it, but it only half-works (I came across pypa/setuptools#175 and other niceties).

[hidmic]

This doesn't work either... pkg_resources picks up egg links, but importlib.metadata doesn't 🤯

I'm starting to consider rolling back and simply dropping a flat egg in the install space. It's a hack, but Python's packaging ecosystem is not cooperating.

[hidmic]

Superseded by #326.