Currently, we can provide either a PowerShell data file or a
hashtable to the settings parameter. But we would like users to
take advantage of the builtin setting files {PSGallery,DSC,etc}
by providing only their names as values. This provides a easier
way to use these builtin setting files.

Ideally we should use HashtableAst.SafeGetValue() but that API is available only on PSv4 and onwards. As we support PSv3, we need our own implementation. And since we do not want any discrepancy in behavior across different PS versions, we use this custom implementation for all PS versions.

Also changes the namespace from `Microsoft.Windows.PowerShell.ScriptAnalyzer.Generic.Settings` to `Microsoft.Windows.PowerShell.ScriptAnalyzer.Settings`.