Fix using directives for multiple Scala snippets in a Markdown file and for plain scala code blocks

[Gedochao]

Version(s)
nightly

Describe the bug
Using directives are only correctly handled in the first scala raw or scala test snippet.
Also, plain scala code blocks don't handle using directives correctly at all.

Known workarounds
Just put all the using directives in a single scala raw code block

To Reproduce

# Faulty snippet
```scala
//> using lib com.lihaoyi::os-lib:0.7.8
println(os.pwd)
```

Expected behaviour
using directives should be handled correctly in each snippet, even when declared within the bounds of a single Markdown file.