Installed app skips args parsing

If I launch this app with full coordinates it works as expected:

```bash
► cs launch io.get-coursier.sbt:sbt-runner:latest.stable -- -help
Usage: sbt [options]
...
```

But running it with just `sbt` skips the args parsing part:

```bash
► cs launch sbt -- -help
[info] welcome to sbt 1.4.0 (Oracle Corporation Java 1.8.0_192)
...
[error] -help
[error] ^
```

or with `cs install`:

```bash
► cs install sbt
► which sbt
~/Library/Application Support/Coursier/bin/sbt
► sbt -help
[info] welcome to sbt 1.4.0 (Oracle Corporation Java 1.8.0_192)
...
```

Reproduce the difference between `cs launch io.get-coursier.sbt:sbt-runner:latest.stable -- -help`, `cs launch sbt -- -help`, and an installed `sbt -help`. Start by tracing the argument-processing entry point for installed and unqualified apps. Done means `-help` is forwarded and parsed consistently in both launch modes, with the existing behavior preserved for fully qualified coordinates.

Written by the indexing model from the issue text.