feat(rules_toolchain): add `:entrypoint` target to local repositories

This is important to provide a consistent target for resolving hermetic tools in repository rules.

For example, one can use a local tool as so:

```py
local(name = "echo")
something(echo = "@echo//:entrypoint")
```

Later, one can switch out the local entrypoint for a hermetically downloaded tool.

In the future, we may provide a `toolchain_entrypoint` repository rule that provides an
entrypoint that understands how to correctly setup `LD_LIBRARY_PATH` (and similar).

This is helpful when using pre-built binaries from package repositories that are linked
dynamically:

```py
download_deb(name = "unionfs", ...)
download_deb(name = "fuse", ...)
toolchain_entrypoint(
    name = "unionfs",
    libraries = [
        "@fuse//:usr/lib/libfuse.so",
    ],
)
```