Move mill.BuildInfo tomill.main.BuildInfo

[lihaoyi]

We currently encourage import mill._, which causes problems when people want to also import mill.contrib.buildinfo.BuildInfo. There's a workaround to import mill.{BuildInfo => MillBuildInfo, _}, which we do in several places in our own codebase, but it's a pain in the neck and an unnecessary hurdle for users to cross. The majority of users would not need to access mill.BuildInfo at all, so putting it in the default namespace just adds clutter.

The mill.BuildInfo is used throughout mill.scalalib and mill.scalajslib and so on, but is not used in main/, so I put it in as downstream a module as possible which turns out to be mill.main.BuildInfo

[lefou]

Change looks ok-ish. I think having these constant defined earlier in the dependency chain is a plus, but I also see the actual collision. But now, that we identified the root issue, a plugin and it's product (a generated class) sharing the same name, we should try to fix it there. Or at least try to. Could we rename the plugin to e.g. GenBuildInfo if we want stick with the generated BuildInfo name?

[lihaoyi]

Sounds good. I'll merge this first, and open a follow up to possibly rename BuildInfo