fix  Issue 11546 - string import dependency failure

This patch adds writing string imports to the dependency file with the keyword "string" instead of the protection.

I also recommend to undeprecate the -deps=file option, as the alternative option to print to stdout is no good for tooling support, any other output can ruin it.

https://d.puremagic.com/issues/show_bug.cgi?id=11546