oracle: several major improvements

Features:

  More robust: silently ignore type errors in modes that don't need
  SSA form: describe, referrers, implements, freevars, description.
  This makes the tool much more robust for everyday queries.

  Less configuration: don't require a scope argument for all queries.
  Only queries that do pointer analysis need it.
  For the rest, the initial position is enough for
  importQueryPackage to deduce the scope.
  It now works for queries in GoFiles, TestGoFiles, or XTestGoFiles.
  (It no longer works for ad-hoc main packages like
  $GOROOT/src/net/http/triv.go)

  More complete: "referrers" computes the scope automatically by
  scanning the import graph of the entire workspace, using gorename's
  refactor/importgraph package.  This requires two passes at loading.

  Faster: simplified start-up logic avoids unnecessary package loading
  and SSA construction (a consequence of bad abstraction) in many
  cases.

  "callgraph": remove it.  Unlike all the other commands it isn't
  related to the current selection, and we have
  golang.org/x/tools/cmdcallgraph now.

Internals:

  Drop support for long-running clients (i.e., Pythia), since
  godoc -analysis supports all the same features except "pointsto",
  and precomputes all the results so latency is much lower.

  Get rid of various unhelpful abstractions introduced to support
  long-running clients.  Expand out the set-up logic for each
  subcommand.  This is simpler, easier to read, and gives us more
  control, at a small cost in duplication---the familiar story of
  abstractions.

  Discard PTA warnings.  We weren't showing them (nor should we).

  Split tests into separate directories (so that importgraph works).

Change-Id: I55d46b3ab33cdf7ac22436fcc2148fe04c901237
Reviewed-on: https://go-review.googlesource.com/8243
Reviewed-by: David Crawshaw <crawshaw@golang.org>
73 files changed