Eliminate YangModelDependencyInfo

Eliminate YangModelDependencyInfo

ModuleImport is a completely different concept to what we want to
express.

We already have SourceIdentifier, which matches the fields, but
unfortunately it has slightly different co-notations -- for a dependency
a missing revision says it is a wildcard.

We therefore create a dedicated construct, SourceDependency, which
really is a model.api.source contract.

This clarifies a lot of details about SourceInfo and DependencyResolver,
leading to complete elimination of YangModelDependencyInfo, making
DependencyResolve work on SourceInfo.

The extraction logic is hosted in YangIRSourceInfoExtractor, but we
expect it to mvoe to YangIRSchemaSource, once we have dealt with the
code structors within yang-parser-rfc7950.

JIRA: YANGTOOLS-1150
Change-Id: I913a03d2b2486e566d06f11b7f5f0d496f30c98f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>