Especially disabling legacy runfiles.

`use_repo_rule` is available in `6.4.0` and above.

It _hugely_ simplifies what we want to do.

That should be the new minimum version for our rulesets.

As of writing, `6.4.0` is not released. The release candidates do not have
the `use_repo_rule` added, yet. Most rules use `last_rc` Bazel version.

`visibility` is available in 6.0+.

Sebastian Birunt authored Oct 05, 2023 and Matthew Clarkson committed Nov 15, 2023

Adds an tag class that can select a repository based on the current local machine triplet.

For some reason, the `triplet:test` target creates all the Posix shell
scripts for each triplet.

We need a more efficient `triplet` rule implementation.

We also need to figure out why every single triplet is getting resolved.

It is almost impossible to correctly transfer all the local files necessary
for a local binary to the remote service.

For safety, constraint all local toolchains to run on the local client until we
have more time to validate that local toolchains work remotely.

This is certainly possible: C/C++ toolchain define all the local files needed
for remote execution.

We will need to expose the `data` attribute through so that arbitary files can
be globbed and sent along with the binary.

Bazel has conflicting nomenclature here.

They use host machine, in particular `--host_platform` but they also use
local machine when refering to remote execution.

It makes sense to align on one, local seems sensible as that works in both
non-remote-execution and remote-execution conversations.

Allows easy testing of a toolchain binary.

This allows rules that need to propagate multiple files.

`sh_binary` is one example of this.

_Technically_, this should be possible by propagating runfiles
but the toolchains are needed at build time, not runtime.

Importantly, includes the `executable = True` keyword.

Allows usage of `--host_platform=@rules_toolchain//toolchain/platform:host`
to get `libc` aware host constraints.

They are not _real_ constraints but `config_settings` which
can be used in a `select` to switch on the triplet.

The "constraint" is actually a `config_settings` that combines
all of the host constraints but can be used in a `select`.

A simple implementation of `toolchain_local_binary`.

This can be used to create hermetic toolchains

This patch removes an extra layer of indirection in toolchains.

The `toolchain/local/binary` rule sets up the `TemplateVariableInfo`,
`ToolchainInfo` and `DefaultInfo`.

The newly renamed `resolved` rule just forwards on the created
providers after the toolchain resolution has happened.

This removed the need to create an extra `info` rule
to describe the toolchain