This is a repeat of an answer I wrote in the comments. I'm just placing it here as an official answer.
The Forbidden Forest should canonically be considered west of Hogwarts because, as DVK quotes in the question, the sun sets behind the Forbidden Forest. As per common knowledge, the sun rises in the east and sets in the west. The books are the most reliable sources of canon we have. As The Giant of Lannister puts it in the comments, the map was hand-drawn by J.K. Rowling in "just a few minutes" probably without direct reference to her books. It is very much possible that Rowling may have made a continuity error in drawing this map. The biggest indicator of this is evidenced in her positioning of the lake 'behind' the castle i.e. on the opposite side of the double front door entrance.
In Chapter 15, Beauxbatons and Durmstrang, of Harry Potter and the Goblet of Fire the Hogwarts students await the arrival of the delegations from Beauxbatons and Durmstrang on the front lawn:
When the bell rang early, Harry, Ron and Hermione hurried up to Gryffindor Tower, deposited their bags and books as they had been instructed, pulled on their cloaks and rushed back downstairs into the Entrance Hall. The Heads of houses were ordering their students into lines
... "Follow me, please," said Professor McGonagall, "first-years in front ... no pushing ..."
They filed down the front steps and lined up in front of the castle
(pg 212, Bloomsbury Edt.)
So, from this position, virtually on the cusp of the bottom-most front step, it would not be possible for the students to see the Durmstrang ship arrive in the middle of the lake - if the lake were on the other side of the castle.
"The lake!" yelled Lee Jordan, pointing down at it. "Look at the lake!"
From their position at the top of the lawns overlooking the grounds, they had a clear view of the smooth black surface of the water ... Some disturbance was taking place deep in the centre ... and then, out in the very middle of the lake, a whirlpool appeared ... Slowly, magnificently, a ship rose out of the water, gleaming in the moonlight.
This quote explicitly places the lake within a field of vision from the front steps.
Now, although I have argued that the map is incorrect, I did notice something new whilst re-reading the first quote that I think must be mentioned:
(Pg 212 again) They filed down the front steps and lined up in front of the castle. It was a cold, clear evening; dusk was falling and a pale, transparent-looking moon was already shining over the Forbidden Forest ... "Nearly six," said Ron, checking his watch and then staring at the drive which led to the front gates.
This last quote presents a new problem, I think. If it is dusk, around 6pm, then the moon should be rising over the east, right? According to this scene, east happens to be over the Forbidden Forest - which ties in with the map Rowling has drawn - but this contradicts the sunset in Prisoner of Azkaban (see DVK's quote in the question), which also happens over the Forest, suggesting the Forest's position as to the west of Hogwarts.
I think Rowling may have made some continuity mistakes with the Hogwarts grounds that we can't really make sense out of. It's no big deal, really, it's similar to the mix-up with every September 1st (start of the school year) happening to land on a Monday, the start of the school week; but for the purposes of this discussion, it makes answering the question difficult.