This change should make it less confusing where utility code comes from.
Having it in two places made sense when we were debating whether to publish
Sandbox2 separately, but not any longer.
Follow-up changes will move `sandbox2/util.h` and rename the remaining
`sandbox2/util` folder.
PiperOrigin-RevId: 351601640
Change-Id: I6256845261f610e590c25e2c59851cc51da2d778
Put duplicated code inside the transaction, which is used by both tests and raster_to_gtiff
Removed <filesystem> header, reimplemented one of its utilities for file checking
Code style changes
Replaced .data() with .c_str() for std::string
Updated README to show how to build both GDAL and PROJ inside the build folder and how to link them to the sandbox
