feat(local_which): make local binaries non-mandatory

This allows the implicit `:resolved` target to be used even when the
local binary is not provided.

Technically, we can avoid that because one could set up both `local`
_and_ `resolved` repository rules. That is a boilerplate pain though.

This moves the error to execution time which is a bit of a pain.

Previously, because the repository was only resolved when a target
explicitly requested the local toolchain the error would be caught
before execution time. However, moving the convenience `:resolved`
target into the `local` repository nullifies that because the
repository needs to be resolved for _any_ toolchain because the
`:resolved` target is re-used.

The trade-off here seems worth it for simple `MODULE.bazel`
repository definitions.