This PR fixes Lake's handling of a module system `import all`.
Previously, Lake treated `import all` the same a non-module `import`,
importing all private data in the transitive import tree. Lake now
distinguishes the two, with `import all M` just importing the private
data of `M`. The direct private imports of `M` are followed, but they
are not promoted.
This also fixes some other Lake bugs with module system imports that
were discovered in the process.
