hack: permit `foo/bar/baz:version` in toolchain names

[eric-wieser]

This was legal (accidentally?) in previous elan version, and let me smuggle Bazel labels into the lean-toolchain files.

Look, my setup works for me. Just add an option to reenable ...

Fixes #139

[Kha]

I can't accept this if it just throws away parts of the regex match. But for local toolchains, even the version match doesn't make much sense. How about changing the code so that we simply accept name as-is as a local toolchain if it exists and only do the matching otherwise?