Avoid modifying alias targets in the installed CMake config - #192

An existing libtcod::libtcod alias makes find_package(libtcod) fail because the installed config calls target_include_directories on that alias.

Remove the redundant include-directory mutation. The exported target already carries the install include paths. Keep LIBTCOD_INCLUDE_DIR, LIBTCOD_LIBRARY, dependency lookup, and required-component checking unchanged.

Add a downstream CMake consumer and a separate Linux CI job for static/shared packages. The tests relocate the installation before compiling, linking, and running a consumer; check that repeated find_package does not change include directories; and check that an existing alias is preserved. The package job disables optional SDL/zlib/PNG/Unicode dependencies to isolate the config/export behavior.

Validation on Linux (GCC 12, CMake 3.25):

• Unmodified upstream reproduces the reported alias error. The new repeated-load check also fails on upstream and passes with the fix.
• Both new CI command sequences pass locally, including the consumer runtime check.
• Full portable suite: 78 cases passed, with all default optional features enabled; interface-header verification passed.
• SDL3-enabled shared build with PNG/Unicode disabled: 59 cases and the relocated consumer passed.
• Repository pre-commit hooks passed for all four changed files.

GUI-dependent tests tagged [!nonportable], Windows, and macOS were not run locally.

Fixes #191