[loader] resolve symlinks when looking for an image in the loaded_images cache

[loader] resolve symlinks when looking for an image in the loaded_images cache

Consider the old code (which just used mono_path_canonicalize)

Suppose foo.dll is a symlink to bar.dll

Suppose we've try to load foo.dll the first time:
do_mono_image_open is passed "foo.dll" it then sets image->name to "bar.dll"
which is then used by register_image to do a lookup and then inserts the
mapping from "bar.dll" to the new image (call it Image1).

Suppose we try to load foo.dll a second time:
we check for "foo.dll" in the loaded_images cache and get NULL.
So we call do_mono_image_open ("foo.dll", ...) again, and again image->name is
set to "bar.dll".  (Call this Image2)
We call register_image again which looks up bar.dll and gets Image1.  So it
increments the refcount for Image1 and unloads Image2 and so
mono_image_open_a_lot returns Image1.

With the new code:

The first time we try to load foo.dll everything is the same.  The second time
we try to load foo.dll, we resolve the symlink from "foo.dll" to "bar.dll" in
mono_image_open_a_lot and return Image1 directly from loaded_images.