cmd/go/internal/doc: clean up after merge with cmd/internal/doc

This is done in a separate CL to reduce the diffs from the previous CL.
Merge the main.go and doc.go files, and isolate the bootstrap-tagged
code to one file.

For #74667

Change-Id: I11bf0aa18beeb898937135f49f473c1ba1b7e756
Reviewed-on: https://go-review.googlesource.com/c/go/+/689875
Reviewed-by: Michael Matloob <[email protected]>
Auto-Submit: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: matloob

src/cmd/go/internal/doc/dirs.go

@@ -2,8 +2,6 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-//go:build !cmd_go_bootstrap
-
 package doc
 
 import (