Add experimental_expose_build_files flag

[sewerynplazuk]

Attempt to solve #932.

Following the example given in the linked issue, with the new flag experimental_expose_build_files, it is possible to granularly expose the build files of individual packages, as an example:

swift_deps.configure_package(
    name = "swift-nio-transport-services",
    experimental_expose_build_files = True,
)
swift_deps.configure_package(
    name = "swift-protobuf",
    experimental_expose_build_files = True,
)
use_repo(
    swift_deps,
    "swift_deps_info",
    "swift_package",
    "swiftpkg_swift_nio_transport_services",
    "swiftpkg_swift_protobuf",
)