Export Swift binary from standalone toolchain

[adincebic]

Allowing Swift binary to be invoked from things like rules_swift_package_manager.
Currently the rules_swift_package_manager invokes swift package during loading phase which fails with standalone Swift toolchain because it can't find swift on the path.

To work around that, we are allowing Swift binary to be injected by the consumer:

swift_deps.from_package(
    swift = "//third_party/swift:Package.swift",
    resolved = "//third_party/swift:Package.resolved",
    swift_executable = "@swift_toolchain_ubuntu22.04//:usr/bin/swift",
)

Referencing @swift_toolchain_ubuntu22.04//:usr/bin/swift currently fails because the swift binary needs to be exported.

Example usage in cgrindel/rules_swift_package_manager#2511