Sort target frameworks of NuGet lock files

What does this MR do?

Sort "targets" when processing NuGet lock files in order to get a deterministic list of dependencies.

When resolving dependencies and collecting dependencies, use a dependency map that maps a package name to the package version defined in the last target where this package has been seen.

The returned packages are unchanged.

NOTE: This is no more than a workaround and ideally Gemnasium would generate one dependency graph per NuGet target. This is out of scope though.

NOTE: Unfortunately there's no proper way to merge dependency graphs because a package with a given name and version can't be repeated in the returned packages.

What are the relevant issue numbers?

gitlab-org/gitlab#328451 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated for GitLab EE, if necessary
• Documentation created/updated for this project, if necessary
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Job definition updated, if necessary
• Conforms to the code review guidelines
• Conforms to the Go guidelines
• Security reports checked/validated by reviewer