…the target

Since swiftlang#7164, dependencies of command plugins are once again being
built for the _target_ rather than the host.   This causes problem
when cross compiling because the host needs to be able to run the
plugin dependencies, but finds target binaries instead.

This problem was fixed before in swiftlang#6791 by forcing command plugin
dependencies to be built for the host by overriding the default
build parameters in swiftTool.createBuildSystem().  The same solution
still works in this commit, but a better long-term option would be
to rework BuildOperation.plan() to handle command plugin dependencies
specially, as it already does for build plugin dependencies.

At present, BuildOperation.plan calls graph.invokeBuildToolPlugins to
process sources.  invokeBuildToolPlugins finds all build tool dependecies
and builds them separately, using a specially-created BuildOperation instance:

    https://github.com/apple/swift-package-manager/blob/34efc0bfe9d40d9a019644ac8fcd0b852c491dfe/Sources/SPMBuildCore/Plugins/PluginInvocation.swift#L409

There is no equivalent step for command plugin dependencies, so
they are built for the host architecture.   Ideally we should rework
BuildOperation.plan to build command and build plugin dependencies
in the same way.

This integration test checks that any targets depended on by a
command plugin are built for the host, not for the target.

* A new CommandPluginTestStub plugin has a dependency on a target
  executable which will be built automatically when the plugin is
  run.   The test checks that the dependency is built for the host
  architecture, no matter which target architecture is selected
  using '--triple'.

* The plugin also asks SwiftPM to build the 'placeholder' main
  target.   The test checks that the dependency is built for the
  target architecture.

The test is restricted to macOS because we can be sure of having
a viable cross-compilation environment (arm64 to x86_64 and vice
versa).  The standard Linux build environments can't cross compile
to other architectures.