Allow run with no sources / replacing cs launch

[kubukoz]

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

scala-cli is almost capable of replacing cs launch from Coursier: the only thing that appears to be missing, in order to be able to run an arbitrary application from dependencies, is that scala-cli run requires at least one source file / directory.

For example:

scala-cli run --dep software.amazon.smithy:smithy-cli:1.45.0 . --main-class software.amazon.smithy.cli.SmithyCli

Describe the solution you'd like

If dependencies are provided, relax the requirement of having to pass a file/directory.

In addition, there are a couple nice-to-haves:

• discover main classes from dependencies, maybe with a flag?
• if no sources are passed, don't start a compilation server (assume --server=false?)

Describe alternatives you've considered

• Using cs launch as usual
• Using a dummy empty directory as the sources

Additional context

This would be nice to enable the usage of code generators, such as smithy4s, without waiting for #610. For example, at the moment:

scala-cli run --scala 2.13 --dep com.disneystreaming.smithy4s::smithy4s-codegen-cli:0.18.15 . --main-class smithy4s.codegen.cli.Main --server=false -- version

which isn't terrible, but still requires that dummy directory (.).