Issues in implementation of ILocalizationService

[MatteoPiovanelli-Laser]

related to #7386 and #7352.
As discussed with @sebastienros, I am opening an issue about the methods from ILocalizationService.

Method: GetLocalizedContentItem(IContent content, string culture, VersionOptions versionOptions)
(versionOptions is optional because there is a version of the method without that parameter)
Expected behaviour: Call a "LocalizationSet" the set of all contents that are localizations one of the other. I would expect this method to return the item from content's LocalizationSet whose Culture property matches the one given in the string culture.
Actual behaviour: That only happens if content is the MasterContentItem for its set. If that's not the case the method returns:

• null if content's Culture does not match culture.
• content otherwise.
  This is all down to the query used in this method:

return _contentManager
    .Query<LocalizationPart>(versionOptions, content.ContentItem.ContentType)
    .Where<LocalizationPartRecord>(l =>
        (l.Id == content.ContentItem.Id || l.MasterContentItemId == content.ContentItem.Id) // <-- I THINK THE ISSUE IS HERE
        && l.CultureId == cultureRecord.Id)
    .Slice(1)
    .FirstOrDefault();

As things are now (with #7386), given var localized = content.As<LocalizationPart>();, I would change the line I marked above to something like:

(l.Id == content.ContentItem.Id || l.MasterContentItemId == content.ContentItem.Id || (localized.MasterContentItem != null && l.MasterContentItemId == localized.MasterCotnentItem.Id))

I think that change would allow to at least look into the set of items that share content's MasterContentItem.

Method: GetContentCulture(IContent content)
Actual behaviour: This method returns content's culture if content has a LocalizationPart. It returns the Site's culture otherwise (which is debatable).

Method: SetContentCulture(IContent content, string culture)
Actual behaviour: This method is fine. If content has a LocalizationPart, it will wet its culture.

Method: GetLocalizations(IContent content, VersionOptions versionOptions)
(versionOptions is optional because there is a version of the method without that parameter)
Expected behaviour: I would expect this method to return what we called "LocalizationSet" above.
Actual behaviour: This mostly works as expected. However, because of #7386, LocalizationSets are fragmented whenever some localizations are not created directly starting from a common MasterContentItem.