cmacro_import doesn't handle directories correctly.

[setupminimal]

I was testing out Magma, and I had this simple test code:

cmacro_import "fn/lazy.c"

int main(void) {
  return force (delay 1);
}

But cmc gave the following error:

error opening #P"/path/to/pwd/magma/lambda.c": No such file or directory

lazy.c was requiring lambda.c, but instead of looking in the same directory as lazy.c, cmacro was looking in the current working directory, which of course didn't have lambda.c in it.