Significantly improve CPU and memory usage when not using SSA-powered linters.
Improve readability of go/packages errors.
Improve debugging capabilities and write doc about debugging.
Preprocessed files like .qtpl.go quicktemplate Go files can have
//line directives. They map to a source .qtpl file.
This commit fixes linting of such files:
1. don't fail on AST cache loading
2. output Go filename not .qtpl or similar
Also, here we update golint to the upstream version.
Relates: #316, #466, #467, #468
