Holds toolchains available for a particular exec group. Toolchain targets are accessed by indexing with the toolchain type, as in
ctx.toolchains["//pkg:my_toolchain_type"]. If the toolchain was optional and no toolchain was resolved, this will return
None. Accessing toolchains of an aspect or rule via
ctx.toolchains returns the indexed toolchain as a
ToolchainInfo provider. While when using aspects,
ToolchainContext is also used to hold the toolchains of the base target. It can be accessed by
ctx.rule.toolchains["//pkg:my_toolchain_type"] and it returns the list of providers resulted from applying the aspects on these toolchain targets.
