cmd/godoc: use go/build to determine package and example files

Also:
- faster code for example extraction
- simplify handling of command documentation:
  all "main" packages are treated as commands
- various minor cleanups along the way

For commands written in Go, any doc.go file containing
documentation must now be part of package main (rather
then package documentation), otherwise the documentation
won't show up in godoc (it will still build, though).

For commands written in C, documentation may still be
in doc.go files defining package documentation, but the
recommended way is to explicitly ignore those files with
a +build ignore constraint to define package main.

Fixes #4806.

R=adg, rsc, dave, bradfitz
CC=golang-dev
https://golang.org/cl/7333046

1 files changed, 3 insertions, 1 deletions

diff --git a/src/cmd/8l/doc.go b/src/cmd/8l/doc.go
index 12301d4f2e..ff06bc3761 100644
--- a/src/cmd/8l/doc.go
+++ b/src/cmd/8l/doc.go
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build ignore
+
 /*
 
 8l is the linker for the 32-bit x86.
@@ -10,4 +12,4 @@ The $GOARCH for these tools is 386.
 The flags are documented in ../ld/doc.go.
 
 */
-package documentation
+package main