Always symlink TARGET_{LINKER,SONAME}_FILE on libraries

[cottsay]

During non-symlink install, these files are copied to the install prefix by default. We should do the same for symlink installs if they're specified.

Reproducer:

1. $ git clone https://gist.github.com/4d4ebcbd310e6337527e3126ac515298.git src/testpkg
2. $ colcon build --packages-select testpkg --event-handler console_direct+

Expected:

...
-- Install configuration: ""
-- Execute custom install script
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/libtestpkg.so.1.2.3
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/libtestpkg.so.1
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/libtestpkg.so
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/libtestpkg.a
-- Symlinking: /home/cottsay/rolling_ws/install/testpkg/testpkg
...

Actual:

...
-- Install configuration: ""
-- Execute custom install script
-- Up-to-date symlink: /home/cottsay/rolling_ws/install/testpkg//libtestpkg.so.1.2.3
-- Up-to-date symlink: /home/cottsay/rolling_ws/install/testpkg//libtestpkg.a
-- Up-to-date symlink: /home/cottsay/rolling_ws/install/testpkg//testpkg
...

Non-symlink:

...
-- Install configuration: ""
-- Installing: /home/cottsay/rolling_ws/install/testpkg/lib/libtestpkg.so.1.2.3
-- Installing: /home/cottsay/rolling_ws/install/testpkg/lib/libtestpkg.so.1
-- Installing: /home/cottsay/rolling_ws/install/testpkg/lib/libtestpkg.so
-- Installing: /home/cottsay/rolling_ws/install/testpkg/lib/libtestpkg.a
-- Installing: /home/cottsay/rolling_ws/install/testpkg/bin/testpkg
-- Set runtime path of "/home/cottsay/rolling_ws/install/testpkg/bin/testpkg" to ""
...

Aside:
I'm not quite to the bottom of it, but installing with EXPORT seems to deactivate the symlink process for targets, which is probably why this bug went unnoticed for so long.

[sloretz]

I'm not quite to the bottom of it, but installing with EXPORT seems to deactivate the symlink process for targets, which is probably why this bug went unnoticed for so long.

The code for symlink install falls back to the built in _install command when the EXPORT keyword (and some others) are used.

ament_cmake/ament_cmake_core/cmake/symlink_install/ament_cmake_symlink_install_targets.cmake

Lines 31 to 32 in 446e3ed

	set(unsupported_keywords
	"EXPORT"

[sloretz]

It looks like whether these extra files get installed depends on the NAMELINK_ prefixed arguments to install(): https://cmake.org/cmake/help/latest/command/install.html#signatures

What do you think about deferring to the native install command if any of the NAMELINK_ arguments are given?

EDIT: looks like we already do that:

ament_cmake/ament_cmake_core/cmake/symlink_install/ament_cmake_symlink_install_targets.cmake

Lines 42 to 43 in 446e3ed

	"NAMELINK_ONLY"
	"NAMELINK_SKIP"