lint,cmd/kfulint: record types info and parsed ast files inside context

[quasilyte]

Currently, ast.ParseDir results are ignored.
It returns multiple packages, usually it's target package (same name as containing directory name) only,
but sometimes there is also testing package with _test suffix.

Proposed plan:

1. Collect parsed packages.
2. Iterate over these packages, init context with current package files and types info.
3. Run checkers over that package with properly initialized context.
4. Repeat until there are no packages left.

Note: we may reject multi-package folders for now.
In other words: it's OK to exit if len(packages) != 1 and that package name does not
match target package name. "proper" multi-package handling may be added later.
This can simplify initial implementation significantly.

lint.Context needs additional fields.
It definitely needs *types.Info and []*ast.File. Maybe something else.

[fexolm]

Is it necessary for us to handle packages? Right now we don't have ( in plan ) any linters that would need any package information ( as far as i know ). So, maybe we would process every file independently? (Maybe not use parsedir at all, but use parsefile)
If it is possible ofc.

[quasilyte]

Yes, we can ignore that detail and operate on file level.

[quasilyte]

#7 solved that.