Extensions: Permit selective import from subfolders

[alex-kulcsar]

Is your feature request related to a problem? Please describe.

To import an extension, a repository needs to be dedicated just to the extension.

Describe the solution you'd like

Allow extensions to be created within subfolders of a repository and allow selective import by subfolder into a tutorial.

In other words, allow something like the following construction in tutorials:

```package
x1=github:author/repo/x1
x2=github:author/repo/x2
```

Would be nice to also allow a URL like https://github.com/author/repo/x1 to be used to import an extension into MakeCode projects, too.

Additional context

Creating public repositories can be a hassle for some organizations. Allowing multiple extensions to reside within a single repo can overcome this burden for these organizations.