TVDB search fix

[Larvitar]

Copy of #4159

Fixes several issues with TVDB plugin series search. TVDB would never run a "name only" search in GetSearchResults and GetMetadata for short series names would often return wrong results.

Changes

1. IsValidSeries always returned true for SeriesInfo object in GetSearchResults.
2. Never include year in search name as TVDB tends to get confused by it (see #4132).
3. Improve search result sorting.

Issues
Fixes #4132

[crobibero]

Sorry, I snuck in a change while you were migrating the PR! Thankfully it's only a style fix :)

[Larvitar]

I should have checked it though. I'll resolve it in a minute :)

[Larvitar]

BTW. I've noticed you've changed some instances of TvdbPlugin.ProviderName to MetadataProvider.Tvdb in d4b5b55 . Was this intentional?

[crobibero]

Yes, as we move providers away from server I've been setting the ProviderName to something that doesn't rely on server.
It's the same string so the end result will be the same

[Larvitar]

That's why I'm asking. MetadataProvider.Tvdb relies on the server. You've changed it to TvdbPlugin.ProviderName in the initial version of plugin but reverted 2 lines in d4b5b55 . Now we use both values in TvdbSeriesProvider.cs.

[crobibero]

Fixed in master, thanks for catching that!