Don't fail, if a build.{sc,mill} is found in sub-directories

[lefou]

Mill currently fails with an error message when it detects a build.sc or build.mill file in a sub-directory. This isn't an issue in any way, so it should just ignore it. Printing a warning message might be an option, but I currently don't see why this is even a worth a warning.

IMHO the most likely situation where users might want to be warned is, when we detect a package.mill file next to an build.mill. This means, that a directory is a root project as well as a sub-project. This is not an issue per se, but package.mill files further down the directory hierarchy might be ending up in both Mill projects, which can be wanted or not.

• See also dicsussion New features and breaking changes in 0.12.x #3349 (reply in thread)