Allow setting location of Dependencies.scala explicitly

[jatcwang]

Many multi-module projects have the following directory structure

build.sbt
project
  Dependencies.scala
modules
  proj1
    src
      ...
  proj2
    src
      ...

But my understanding of the Dependencies.scala location detection only checks the current project's base dir and parent. i.e. it's designed for this structure

build.sbt
project
  Dependencies.scala
proj1
  src
    ...
proj2
  src
    ...

Are you happy with an sbt setting that allows you to set the location of Dependencies.scala explicitly? We can keep the default behaviour the same