Improving test coverage of determinePrimaryCandidate

Added tests checking the behaviour of autowiring by type when the @Primary
annotation is present on the class. Fixed also some inconsistencies in the
code.
Add support for @Priority

This commit adds support for @Priority to filter multiple candidates
for autowiring. When multiple candidates are available for a given
bean, the bean annotated with @Primary is used. If none exists, the
one with the higher value for the @Priority annotation is used. If
two beans have the same priority a NoUniqueBeanDefinitionException
is thrown, just as if two beans are annotated with @Primary.

The underlying code for #getBean and #resolveDependency has been
merged as this feature is available for both dependency injection
and bean lookup by type.

Issue: SPR-10548