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.
Use go/packages instead of x/tools/loader: it allows to work
with go modules and speedups loading of packages with the help
of build cache.
A lot of linters became "fast": they are enabled by --fast now and
work in 1-2 seconds. Only unparam, interfacer and megacheck
are "slow" linters now.
Average project is analyzed 20-40% faster than before if all linters are
enabled! If we enable all linters except unparam, interfacer and
megacheck analysis is 10-20x faster!
