package_manager/__init__.py: add function to give user reason about a missing package

package_manager/__init__.py: add function to give user reason about a missing package

When users specify some package in IMAGE_INSTALL, but get some error
at rootfs time, the user might be confusing. This usually happens
when the user puts a recipe name in IMAGE_INSTALL.

To helper user understand more about what's going on, add a common
function here which makes use of pkgdata data to give the possible
reason about a missing package. This function is expected to be used
by package backends such as rpm.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>