all: fix duplicate package docs and tighten TestPackageDocs

TestPackageDocs walked into directories starting with "." (such as
.claude worktrees) and only logged warnings on duplicate package docs
across files in a directory. Skip dot-directories (which covers the
old .git but also .claude), ignore files with "//go:build ignore" so
command files don't falsely trip the duplicate check, and promote the
duplicate-doc warning to a t.Errorf.

While here, deduplicate the package docs that were previously only
logged: drop the redundant comment from client/systray/startup-creator.go,
move the comprehensive taildrop doc into feature/taildrop/doc.go, and
remove a leftover doc fragment from feature/condlite/expvar/omit.go.

The tstest/integration/vms allowlist is no longer needed since the
//go:build ignore filter now handles its dns_tester.go and udp_tester.go
files generically.

Fixes #19526

Change-Id: Id794d96bd728826a1883a054e4a244f90fa05d3d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/condlite/expvar/omit.go

@@ -3,7 +3,6 @@
 
 //go:build ts_omit_debug && ts_omit_clientmetrics && ts_omit_usermetrics
 
-// excluding the package from builds.
 package expvar
 
 type Int int64

feature/taildrop/doc.go

@@ -1,5 +1,10 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-// Package taildrop registers the taildrop (file sending) feature.
+// Package taildrop contains the implementation of the Taildrop
+// functionality including sending and retrieving files.
+// This package does not validate permissions, the caller should
+// be responsible for ensuring correct authorization.
+//
+// For related documentation see: http://go/taildrop-how-does-it-work
 package taildrop

feature/taildrop/taildrop.go

@@ -1,12 +1,6 @@
 // Copyright (c) Tailscale Inc & contributors
 // SPDX-License-Identifier: BSD-3-Clause
 
-// Package taildrop contains the implementation of the Taildrop
-// functionality including sending and retrieving files.
-// This package does not validate permissions, the caller should
-// be responsible for ensuring correct authorization.
-//
-// For related documentation see: http://go/taildrop-how-does-it-work
 package taildrop
 
 import (