Add toolchain type for Java bootstrap runtime

[fmeum]

The bootclasspath rule in rules_java will soon use this toolchain type instead of the regular Java runtime toolchain type, which naturally carries a constraint on the target platform.

Also adds more detailed explanations of the now three Java toolchain types.

Work towards #17085
Work towards #18265
Work towards bazelbuild/rules_java#64
Split off from #18262

[fmeum]

@bazel-io flag

[iancha1992]

@bazel-io fork 6.4.0

[iancha1992]

@fmeum @hvadehra
Looks like we may need another small commit before cherry-picking this to release-6.4.0 because in the BEGINNING of the "tools/jdk/BUILD.tools" file, we have:

In the master branch:

package(default_visibility = ["//visibility:public"])
load("@rules_python//python:defs.bzl", "py_binary", "py_test")
exports_files([
    "BUILD.java_tools",
    "java_stub_template.txt",
])

In the release-6.4.0 branch:

load(
    "@bazel_tools//tools/jdk:default_java_toolchain.bzl",
    "DEFAULT_TOOLCHAIN_CONFIGURATION",
    "PREBUILT_TOOLCHAIN_CONFIGURATION",
    "VANILLA_TOOLCHAIN_CONFIGURATION",
    "bootclasspath",
    "default_java_toolchain",
    "java_runtime_files",
)
load(
    "//tools/jdk:java_toolchain_alias.bzl",
    "java_host_runtime_alias",
    "java_runtime_alias",
    "java_runtime_version_alias",
    "java_toolchain_alias",
)
load("//tools/python:private/defs.bzl", "py_binary", "py_test")

package(default_visibility = ["//visibility:public"])

The difference is causing the merge conflicts. Can you please take a look and let me know which PR or commit I need? Thanks.

cc: @bazelbuild/triage

[fmeum]

@iancha1992 That commit is very large and can't be cherry-picked, but it also isn't really needed: You can replace

# Used to distinguish toolchains used for Java development, ie the JavaToolchainProvider.
toolchain_type(name = "toolchain_type")

# Used to distinguish toolchains used for Java execution, ie the JavaRuntimeInfo.
toolchain_type(name = "runtime_toolchain_type")

with the lines added in this PR and keep everything before and after as is. If that doesn't work, I can try to submit a PR.

[iancha1992]

@iancha1992 That commit is very large and can't be cherry-picked, but it also isn't really needed: You can replace

# Used to distinguish toolchains used for Java development, ie the JavaToolchainProvider.
toolchain_type(name = "toolchain_type")

# Used to distinguish toolchains used for Java execution, ie the JavaRuntimeInfo.
toolchain_type(name = "runtime_toolchain_type")

with the lines added in this PR and keep everything before and after as is. If that doesn't work, I can try to submit a PR.

@fmeum actually could you submit a PR please?

[fmeum]

@iancha1992 Sure: #19220

[iancha1992]

The changes in this PR have been included in Bazel 6.4.0 RC1. Please test out the release candidate and report any issues as soon as possible. If you're using Bazelisk, you can point to the latest RC by setting USE_BAZEL_VERSION=last_rc.
Thanks!