It doesn't matter.
Parting a package into multiple source files is something for you, to organize your code better. But to the Go compiler it doesn't matter. If you have an unexported identifier in one source file, you can use that in the other source file (same package of course) without any notice (like
import). What matters is that your package will have a dependency on the other package.
Note that it affects compilation efficiency in the slightest way: the compiler will have to process multiple files, create multiple internal data structures (required only for compilation), process the same sections (e.g.
import) multiple times. But this should never affect your judgement to choose 1 file over multiple. Just some self made-up numbers to help understanding it: does it matter if your package with 1 source file compiles in 100 ms or it takes 100.1 ms if it is broken into 5 files? No, obviously.
Going further, compilation time should also not be a factor even if it is much bigger (like 1 sec vs 2 sec). Compilation time and run time efficiency are 2 different things, the latter is what should concern you.