Question on argument passing

I have a script that is run by issuing source script.sh.

The directory structure looks like this:

project_root/
|- script.sh
|- plugins
   |- manifest

script.sh has a function inside that looks like this:

# call additional modification scripts
if [ ! -e "plugins/manifest" ]; then
  printf("Error: Could not find manifest file.")
  return
fi

Because this function uses the relative path of the plugins directory, if I were in working on a file in the plugins directory I could not issue a command like this: source ../script.sh.

Is it possible for the calling command source ../script.sh to pass the relative directory (..) to script.sh so that script.sh could find the plugins directory?