Issue Description:

Package plugin targets can depend on executables, which are then made available to the plugin so it can be looked up with `PluginContext.tool(named🙂`. This doesn't work correctly when the plugin is in one package and the executable is in another, as illustrated by `https://github.com/tonyarnold/swiftpm-buildtool-plugin-examples/tree/swiftlint-example/Example%205%20-%20SwiftLint\` (which I've also replicated in a corresponding `swiftlint-example` at https://github.com/abertelrud/swiftpm-buildtool-plugin-examples).

Possible workarounds include:

1. put the plugin in the same package as the executable (such as in a fork of the swiftlint repo, similarly to the https://github.com/abertelrud/swift-protobuf.git example), or
2. have a dependency on a precompiled binary of swiftlint (using a .binaryTarget), or
3. preinstall swiftlint somewhere where the plugin can find it (e.g. /usr/local/bin)

This should be relatively straightforward to fix in mainline SwiftPM.