Add implicit dependency download support

[jensmueller-com]

This pull request contains the following changes:

• Rename dg.lib.download_manager package to dg.lib.dependency_manager
• Rename DownloadManager class to DependencyManager (not detected by Git, manual comparison required)
• Rename AbstractDownloadManagerPlugIn class to AbstractDependencyManagerPlugIn (not detected by Git, manual comparison required)
• Move logic to obtain the path of the binary provided by a dependency from the DataGateConfigurationManager class to the DependencyManager class
• Add support for implicitly downloading a dependency if the binary provided by a dependency is called but does not exist (dg/lib/dependency_manager/dependency_manager.py) (Fixes https://github.ibm.com/Everest/nimbus-planning/issues/176)

[jensmueller-com]

Eirini (@eirinikalogeiton) reviewed this pull request as part of her review of pull request 98.