Host a toolchains repository

[alexeagle]

Toolchains are like Providers:

• The toolchain_type definition must be canonical. Everyone must agree on the symbol to use, e.g. @someones_repo//toolchains:grep_toolchain
• A concrete toolchain might be defined in one or more rulesets, or end-user monorepos might declare their own (e.g. build from source with specific flags or target platform)
• A toolchain might be consumed in one or more rulesets, which shouldn't have to define it themselves. Many rulesets would like to be able to rely on a hermetic grep executable for example. They could leave it for users to choose one of the concrete toolchain implementations.

Currently the world is broken because rulesets that define a concrete toolchain (e.g. the tar_toolchain in aspect bazel-lib) ALSO define a toolchain_type symbol.

On Slack, @rickvanprim proposed that a new toolchains repo could be created. I think it's a good idea. Note that it must be suitable to be a common dependency of many rulesets, like https://registry.bazel.build/modules/bazel_features

[rickvanprim]

I think there are 3 pieces that would need to be part of a centralized repo, the toolchain_type, any provider definitions that get used in the ToolchainInfo, and the rule that constructs that ToolchainInfo.

[meteorcloudy]

In general, I agree this is the right direction. However, we also need to think about how to migrate existing definitions to this central repo in a less intrusive way for users. For example, when we moved most of java toolchains from bazel_tools to rules_java, we had to leave the java toolchain in bazel_tools to be backwards compatible.

/cc @comius probably has more thoughts on this.