Run all linters per package. It allows unloading package data when it's
processed. It dramatically reduces memory (and CPU because of GC) usage.
Relates: #337
Treat Go source files as plain text files by misspell: it allows detecting
issues in strings, variable names, etc. Also, it's the default mode of
a standalone misspell tool.
Also, implement richer and more stable auto-fix of misspell issues:
now it can fix multiple issues in one line.
