Provide an API for accessing managed versions

This commit adds an API for accessing managed versions. Two methods are
provided, one for accessing versions from global dependency management
and one for accessing versions from configuration-specific dependency
management:

dependencyManagement.versions.getManagedVersion(group, name)
dependencyManagement.forConfiguration(configurationName).getManagedVersion(group, name)

Versions provided by configuration-specific dependency management
reflect the configuration’s hierarchy. For example, the Java plugin's
runtime configuration extends its compile configuration so versions
from dependency management declared on the compile configuration can
be accessed via the runtime configuration, with any dependency
management declared on the runtime configuration taking precedence.
Furthermore, when querying configuration-specific versions, the
versions from global dependency management will be used as a fallback.

Closes #11