Restore Sass dependencies

Sass `@import`s no longer generate the right dependencies in nanoc 3.6.6. This PR fixes that.

nanoc 3.6.6 includes a change that reduces the number of redundant dependencies. However, this change caused *required* dependencies to be removed as well, erroneously assuming they were redundant. The problem existed before 3.6.6, but was masked because the redundant dependencies had the surprisingly beneficial effect of causing Sass files to be recompiled anyway.

The right fix is not to rollback the change introduced in 3.6.6, but rather fix it properly by finding the right imported files. This is what this PR does, and in the process also greatly simplifies the existing code.

This is a fix for #350.

@Leolik @phklevets Can you check whether this fix solves the problem for your sites?

@bobthecow @chriseppstein Can you review this code?