* update staticcheck
Don't fork staticcheck: use the upstream version.
Remove unneeded SSA loading.
* Cache go/analysis facts
Don't load unneeded packages for go/analysis.
Repeated run of go/analysis linters now 10x faster
(2s vs 20s on this repo) than before.
1. Support unparam linter and fix found issues
2. Replace forked mvdan.cc/lint and mvdan.cc/interfacer with the
upstream ones
3. Minimize forked megacheck: move the most of it's code to this repo
4. Use golang.org/x/tools/go/ssa import path instead of custom fork
paths
5. In golang.org/x/tools/go/{ssa,callgraph} use changed code from
honnef.co/go/tools
6. Add megacheck.check-unexported option: it found some issues in
the repo, fixed them all
