lib/packagedata.py: Fix broken symlinks for providers with a '/'

If a package had a provider with a '/' in it (e.g. "/bin/sh",
"/bin/bash", etc.), the generated symlinks were broken due to being at a
hard coded depth. Use oe.path.relsymlink() instead to make a correct
relative symbolic link

Signed-off-by: Joshua Watt <JPEWhacker@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>