toolchains attribute on all rules should merge runfiles from the toolchain into the depending target

[illicitonion]

See bazelbuild/rules_nodejs#2960 (comment)

[illicitonion]

/cc @katre @alexeagle

[katre]

This prevents the need to duplicate a target in both toolchains and tools.

[github-actions]

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[alexeagle]

I think this is still valid.

[katre]

We should rename the existing toolchains attribute, since what it does isn't really part of toolchains.

Unfortunately, that's a major task and no one has time to pick it up.

We should keep this alive, however.