Refactor FindAll to use map of struct{} instead of bool for set.
Package summary code minor tweaks.

Avoid printing dot import.
Use more idiomatic variable name.
Use build.Default.GOPATH over os.Getenv("GOPATH").
Remove unneeded and unused small packages.

Most of these were written when I was just learning Go, and they're
effectively helpers that panic on error. Also, it's better to use
standard library directly as the code is more readable to other people,
so the value offered by these helpers is less than their cost of
unfamiliarity.
Remove ReadAllStdin helper.
Remove GoKeywords helper. This is one of my first Go code ever written.
It still works and returns correct results today. :) But it's not
needed.
Remove GetenvOr helper.
Remove MinInt and MaxInt helpers. Can use github.com/pkg/math instead.
Remove ParseGistId helper. It's used in one place, moved there.
Move ImportPathFound into gist7480523.
Minor code refactor for LineReader.