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
The new version of goimports works 100x faster
with go modules. Also it has some new features:
$ git cherry -v 6c7e314b6563 92cdcd90bf52 | fgrep imports
+ 5bbcdc15656ef390fab5dd6e8daf95354f7171e3 imports: redesign fixImports
+ ee45598d2ff288037f53f9e13ae0b1a6e2165ad5 imports: create named imports for name/path mismatches (again)
+ 4c53570e0460bc32468f75bf9dd71c018d03bfa9 imports: ignore globals in different packages
+ 1d424dbce8dd500e9e449fd3ff9d0668c09e2ae1 imports: clean up customization seam
+ 6a3e9aa2ab7749d72d1006ee484271b4a11f96c2 imports: fix renamed sibling imports
+ 5f4a60f04f23ac48e0665f257413ae3eacf339be imports: fix renamed sibling imports more
+ bbccd8cae4a9a47e0f978e03ff4b5df88a9fde1e imports: use go/packages, support modules
+ d4971274fe382404aee0e8c163af974f2bf738e6 imports: don't remove imports that conflict with globals
