Fix REPL clashing with CWD artefacts

[dwijnand]

This reverts the change that moves all the REPL's artefacts into the
"repl$" package (#12607), moving everything back to the empty package.

Then it fixes the class name and package name shadowing issue, by
simulating what the batch compiler does when it compiles a source file
without a package statement: it considers the sources to be in the empty
package. Then the imports for the previous runs and the user-defined
imports will be in sub-contexts of the empty package.

The reason the working directory artefacts were shadowing the REPL
artefacts was because before the imports were being added to
sub-contexts of the root package, afterwhich the REPL tree was being
wrapped in the empty package. That meant that the classes found in the
empty package (i.e. classfiles in the working directory, or even
directories) had precedence over the wildcard imports for the previous
runs.

The tests now cascade up into contexts that don't have the current
compilation unit stored on them, so I had to safeguard against that in
Typer.

Also, I developed a variant of ShadowingTests, using the batch compiler,
to automate the expectations of how the shadowing should and shouldn't
work.

[SethTisue]

Fixes #12571.

Note that having the REPL use a special REPL package isn't necessarily a bad change, but it seemed cleanest to restore the status quo, since there's no longer a specific motivation to keep the change.

[som-snytt]

We also have to resolve artefacts clashing with artifacts.

This was an interesting discussion which I hope to understand.

If artefacts are namespaced by line objects in the empty package, then it's similar to a Scala 2 idea to make the wrapper objects the package objects for the line. I guess the current rs$line$2 is just a module and doesn't leverage top-level definitions as I had assumed somehow. If REPL supports packagings, it will be interesting that packaged code won't see history, so it will be a private workspace; code in the empty package would see history, as normal.

[griggt]

I agree that there's no need to keep the repl$ package if it's not solving any particular problem.

[griggt]

@dwijnand Nice detective work btw!