[bazel] Update support files.

This adds a MODULE.bazel file matching the sources
in the current repository, and updates BUILD.bazel to
provide definitions that match the ones from the module
of the same name in the Bazel Central Registry.

Note that this does *not* update the sources to the BCR
version, since they include many third-party convenience
libraries too, some of them using a Fuchsia-incompatible
license.

Note that the main target lists are now `zlib` and `z`
to match the BCR module. The target names `zlibonly`
et al are never used in the Bazel part of the
Fuchsia build anyway.

+ Remove invalid definitions, e.g. there is no
  contrib/optimizations/ directory at all so the glob()
  calls were returning empty lists.

After this CL is submitted, it will be possible to add
a new @zlib top-level module that points to
//third_party/zlib.

Change-Id: I47b525dac4a7c2afe44c031deccc6a54e3ef3795
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/zlib/+/1496496
Reviewed-by: Francois Rousseau <frousseau@google.com>