Allow the use of dtk-core as a submodule of another bigger CMake project

Allow the use of dtk-log as a submodule of another bigger CMake project.

The changes:

• rename the export layer-targets to layer-targets-dtkLog to avoid a conflict if both dtk-core and dtk-log are used as subdirectories of the same CMake project,
• and avoid calling find_package(dtkLog) if dtkLog is already a target of the CMake project (in case dtk-log is a sibbling subdirectory of the CMake project).

(Similar to dtk-containers!1 (merged) and dtk-log!11 (closed).)

(cc @maxGimeno)