Fix for plugins that are built as debug or as manualload debug

[gfrolov]

When plugin is built as debug or as manulload debug you would run into this scenarios:

1. Plugins named "pluginName.debug.dll" would get extra ".dll.debug" added because PluginHost thinks that pluginBaseName is just "pluginName"
2. Loading Debug Manualload plugins by name such as "pluginName.debug.manualload.dll" is treated as "pluginName.manualload.debug".

This PR fixes the issue by avoiding appending extra extensions for debug plugins and use the correct order for manualload debug plugins.

[rpavlik]

@mars979 is this still an issue/open? Any response to the review comments?