Implement latest requirements

[helly25]

See issue #473

This supersedes PR #471

Instead of implementing a complex way of predicting the basename, find them by common prefixes in the database we anyway have.

Rather then forcing maintainers to enter more complex information this keeps us at simply adding the versions as generated from the hasher script.

This should also ultimately put #464 and any future occurrence to rest.

There is a new test that verifies which version cannot yet be found. That test shows that past 18.1.0 we can find all configured versions. Also the test shows that the main distribution types can all be found.

That allows support for llvm_version = "latest" which will effectively find the latest LLVM version. For many people this will be the easiest way to get started and solves the issue that not always all arch/os versions have been prepared for all LLVM versions. It also means you do not have to know whether the basename starts with "LLVM" or "clang_llvm", in which order arch and os have to be provided and how the os is exactly specified.

We further add the ability to specify requirements, including minimum and maximum versions as well as version exclusions. That can be easier than figuring out which versions are available for which platform. Further this allows to automatically pick up relevant versions automatically as they become available later. Example:

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm", dev_dependency = True)
llvm.toolchain(
    name = "llvm_toolchain_llvm",
    llvm_version = "latest:>=19.1.6,!=19.1.7,!=20.1.0",
)
use_repo(llvm, "llvm_toolchain_llvm")

The requirements strings mostly follow semver specs and support <, <=, >, >=, == and !=. They only currently work for version components major, minor and patch. Since we do not list other versions in our config that is fine for now.

This PR also adds information for a number of missing distributions.