Starlarkify Xcode rules

[comius]

Rewrite Xcode rules to Starlark:

• xcode_config
• xcode_config_alias  
• xcode_version
• available_xcodes

Those are mostly simple rules that should be straight forward to Starlarkify.

apple_cc_toolchain is extended from cc_toolchain which contains a skyframe restart and is not Starlarkfiable at the moment.

The effort needs to be coordinated with Google:

• [ ] Implement first Starlark version of the rules in builtins (src/main/starlark/builtins_bzl), because this will automatically run unit tests on them; first version should match native version as close as possible
• [ ] Make redirects from rules_apple repository for each rules, pointing to native rules (similar to rules_java or rules_cc)
• [ ] Google should use the redirects internally
• [ ] Copy the rules to rules_apple repository

First step can be done independently of the rest 3 steps.

[comius]

cc @keith @brentleyjones

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