…n be used in downstream bzl_library targets

Downstream rule sets may depend on `@local_config_platform//:constraints.bzl` but when they do there is no way easy way to make that load statement compatible with `bzl_library`. This change makes it possible to use `bzl_library` on starlark code that loads from `@local_config_platform//:constraints.bzl`.

For example,

```
bzl_library(
    name = "local_config_platform_constraints",
    srcs = ["@local_config_platform//:constraints.bzl"],
)

bzl_library(
    name = "platform_utils",
    srcs = ["//lib/private:platform_utils.bzl"],
    deps = [":local_config_platform_constraints"],
)
```

Closes #16665.

PiperOrigin-RevId: 486957479
Change-Id: I328b7a3722aea95b3151ed88f23c277ed4154202