Skip FetchContent if relevant targets already exist

[gareth-cross]

• This change alters the behavior of FindDependencies.cmake such that the PoseLib and colmap repos are not fetched if those targets already exist.
• This makes it easier to build glomap in a larger project where PoseLib and colmap are already CMake sub-projects.

[lpanaf]

Hi, we add flags in the cmake to disable fetch_content. However, it is not always desired to directly use it because it might have version inconsistency

[gareth-cross]

Hi, we add flags in the cmake to disable fetch_content. However, it is not always desired to directly use it because it might have version inconsistency

The goal of this change is to make it so that neither of the following things happens if the target already exists in the CMake project:

• FetchContent should not run.
• find_package should not run.

That way, COLMAP and PoseLib can be built at the same time as GLOMAP in one project.

The current flag will disable the first, but then perform the second (which will fail unless that dependency has been installed somewhere cmake can find it).