[release/10.0.1xx-preview6] Prefer CLI commands to loose files

[github-actions]

Backport of #49485 to release/10.0.1xx-preview6

Customer Impact

The work to add support for running single-file apps from a direct dotnet invocation (dotnet app.cs) introduces a regression in the way built-in commands are executed. A file that matches the detection criteria for single-file apps that has the same name as one of our built-in commands would be prioritized over the built-in command.

Regression

No, this was introduced in p6 and is being fixed in p6, so no users would have encountered this behavior yet.

Testing

Manual testing: what this shows is that while we can run a single-file app with a name matching one of our built-in commands, we can no longer implicitly run it - dotnet run dnx works, but dotnet dnx runs our dnx command as expected.

(dogfood) PS E:\Code\Scratch\foo> dotnet run dnx
    E:\Code\Scratch\foo\dnx.csproj : error NU1102:
      Unable to find package Microsoft.NET.ILLink.Tasks with version (>= 10.0.0-preview.6.25316.103)
        - Found 54 version(s) in nuget [ Nearest version: 10.0.0-preview.5.25277.114 ]

The build failed. Fix the build errors and run again.
(dogfood) PS E:\Code\Scratch\foo> dotnet dnx
Required argument missing for command: 'dnx'.

Description:
  Executes a tool from source without permanently installing it.

Usage:
  dotnet dnx <packageId> [<commandArguments>...] [options]

Arguments:
  <PACKAGE_ID>        Package reference in the form of a package identifier like 'Newtonsoft.Json' or package identifier and version separated by '@' like 'Newtonsoft.Json@13.0.3'.
  <commandArguments>  Arguments forwarded to the tool

Options:
  --version <VERSION>       The version of the tool package to install.
  -y, --yes                 Accept all confirmation prompts using "yes."
  --interactive             Allows the command to stop and wait for user input or action (for example to complete authentication). [default: True]
  --allow-roll-forward      Allow a .NET tool to roll forward to newer versions of the .NET runtime if the runtime it targets isn't installed.
  --prerelease              Include pre-release packages.
  --configfile <FILE>       The NuGet configuration file to use.
  --source <SOURCE>         Replace all NuGet package sources to use during installation with these.
  --add-source <ADDSOURCE>  Add an additional NuGet package source to use during installation.
  --disable-parallel        Prevent restoring multiple projects in parallel.
  --ignore-failed-sources   Treat package source failures as warnings.
  --no-http-cache           Do not cache packages and http requests.
  -v, --verbosity <LEVEL>   Set the MSBuild verbosity level. Allowed values are q[uiet], m[inimal], n[ormal], d[etailed], and diag[nostic].
  -?, -h, --help            Show command line help.

Risk

Low, this change is very scoped and only impacts the single-file-run-from-dotnet use case.