better error reporting for `cabal v2-run -- --opts`

When I run
```
cabal new-run
```
it (usually) picks a sensible target to run. But then I want to pass arguments, like
```
cabal new-run -- --help
```
and I get
```
$ cabal new-run -- --help
cabal: Unknown target '--help'.
There is no component '--help'.
The project has no package '--help'
```
I see how that can be justified if `--` means “treat the rest as non-option arguments”, but I find “pass the rest to the called function” more usefull.

Start by reproducing the reported `cabal new-run -- --help` behavior and compare it with the intended `cabal v2-run -- --opts` syntax. Trace the command-line handling for `--` and verify that arguments after it reach the selected target instead of being treated as Cabal targets; done means the example no longer reports `Unknown target '--help'`.

Written by the indexing model from the issue text.