go.tools/oracle: change -mode argument into subcommand.

e.g. "oracle callgraph <package>"

Also: simplified error handling.
Eliminated oracle.errorf because it prepends "file:line:col: "
to the error message so the main function can't safely prepend "Error: ".
The position wasn't interesting though: it was just -pos, more or less.

R=crawshaw, dominik.honnef, r
CC=golang-dev
https://golang.org/cl/13864044