Document starting programs compiled by scalac in the current working directory

[tgodzik]

Version(s)
1.5.0 follow up from scala/scala3#21432

Describe the bug
If a user compiles using scalac, they can't now run that using scala.

To Reproduce

› scalac hello.scala
› ls -la hello.*
-rw-r--r--@ 1 kieran  staff  758 25 Aug 20:45 hello$package$.class
-rw-r--r--@ 1 kieran  staff  267 25 Aug 20:45 hello$package.class
-rw-r--r--@ 1 kieran  staff  593 25 Aug 20:45 hello$package.tasty
-rw-r--r--@ 1 kieran  staff  912 25 Aug 20:45 hello.class
-rw-r--r--@ 1 kieran  staff   45 25 Aug 20:45 hello.scala
-rw-r--r--@ 1 kieran  staff  502 25 Aug 20:45 hello.tasty
› scala hello
[error]  hello is not a scala sub-command and it is not a valid path to an input file or directory.
Try viewing the relevant help to see the list of available sub-commands and options.
  scala --help

Expected behaviour
Hello should be run. Should we see if hello.class exists in that case and just try to run it? I am not sure if we discussed this breaking (?) change

[Gedochao]

It has been discussed and agreed we would not be supporting this during SIP-46 implementation (this has been a while ago, I'd have to look very deep to link to a discussion).
Scala CLI never tries to include any inputs from the current working directory, nor should it.

The equivalent for this to be working is:

scalac hello.scala
scala run -cp .

Passing the main class may be necessary, in which case:

scalac hello.scala
scala run -cp . -M hello

it is necessary to pass the run command explicitly, as otherwise it is assumed we are launching the REPL (there are no explicit inputs, just a classpath).
We treat passing the current working directory to the classpath as necessary here.

What we should do here is add an entry about it in the migration guide (https://scala-cli.virtuslab.org/docs/guides/introduction/old-runner-migration/), as it indeed seems largely undocumented.

Please note that passing the main class to the old scala script this way is also entirely undocumented, it's very easy to miss documenting these things when they weren't documented in the first place...