[PORT][LG] Redo the default fallback of namespace

[Danieladu]

Issue: microsoft/BotFramework-Composer#3982
C# pr: microsoft/botbuilder-dotnet#4724

Description

Originally, the injection of LG function into Expression require a namespace option. If there is no namespace option, a absolute path is require. It is too strict.

Specific Changes

Loose the namespace fallback policy. The priority order is:

1. If there is a namespace option, use it
2. If there is an un-empty id, parse the file name from the id, and get the namespace
3. If the Id is empty, just drop the namespace and use the template name as the expression name directly.

For example:
If there is an option in LG file, like:

> !# @Namespace = general

general is the namespace, the export function could be presented as general.xxx

If there does not exist a namespace option, use the name of Id by default, for example:
If the id is: ./a.lg -> a is the namespace, a.xxx is the expression name
if the id is: abc -> abc is the namespace, abc.xxx is the expression name
If the is is empty string, just drop the namespace, and use template name directly, xxx is the final expression name