This commit implements a `mode` option that can control how an application is
published with the `dotnet publish` command.
There are three supported modes:
* self-contained: publishes a self-contained application (same as
--self-contained).
* fx-dependent: publishes a framework-dependent application (with an
application host when a runtime is specified).
* fx-dependent-no-exe: publishes a framework-dependent application without an
application host.
The default when publishing without a runtime specified is
`fx-dependent-no-exe`.
The default when publishing with a runtime specified is `self-contained`.
`fx-dependent` requires netcoreapp2.1 or later when a runtime is specified.
The `--self-contained` option is still supported, but is now hidden so that
users will be encouraged to move to the `--mode` option.
Fixes#6237.
bc96902c61