feat: add option to export libraries as cmake targets

[VRichardJP]

Add EXPORT_LIBRARY_TARGETS flag to ament_auto_package() macro.
When the flag is provided, libraries are exported using their cmake target instead of going through ament home-made library export logic.

Although exporting libraries this way is stricter, it makes cmake invocations faster on downstream packages.

Example on Autoware's behavior path planner:

Before: 112s

After: 26s

Note: In my original experimentation, cmake time on this package went down to 17s. I have not checked exactly what is slowing things down yet. ament_ex macros are still way faster (8s on this package).

This feature has been experimented on this autoware branch: autowarefoundation/autoware.universe@f3b6226

Note that this experimental branch also leverages extra macros that are not included in this PR, found here: 450b6de

See #453 for more details

[clalancette]

We are a bit concerned that making things stricter is going to break quite a lot of downstream packages. To get at least some idea of what would happen, here is a full CI run of the ROS 2 core with this in place:

• Linux
• Linux-aarch64
• Windows

[VRichardJP]

Thank you for investigating this.

To be precise, I don't think this PR should break anything dowstream itself. There are only 2 changes here:

• the EXPORT_LIBRARY_TARGETS flag is not enabled by default. Without the flag the code behavior should remain unchanged.
• ament_auto_add_library now use the installed headers instead of the ones in the source directory. Since ament_auto macros have always installed headers I don't think it changes anything.

If this PR was to break anything, I think it would show at build time, no during tests.

[clalancette]

Belated I realized that this will have almost no effect on the ROS 2 core, since we don't use ament_auto there. We would need to do a more comprehensive test of downstream packages that do use ament_auto to see if there is fallout from this.