突然（始于2018年10月17日）我无法在DigitalOcean计算机的libexpat1-dev环境中安装pbuilder。我收到以下错误：
root@build:/# apt-get update
Get:1 http://archive.ubuntu.com/ubuntu xenial-proposed InRelease [260 kB]
Hit:2 http://ppa.launchpad.net/snapcpp/ppa/ubuntu xenial InRelease
Hit:3 http://archive.ubuntu.com/ubuntu xenial InRelease
Fetched 260 kB in 1s (239 kB/s)
Reading package lists... Done
root@build:/# apt-get install libexpat1-dev
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:
The following packages have unmet dependencies:
libexpat1-dev : Depends: libexpat1 (= 2.1.0-7) but 2.1.0-7ubuntu0.16.04.3 is to be installed
E: Unable to correct problems, you have held broken packages.
这似乎真的很奇怪，因为我们看到自Mon, 16 May 2016 12:47:07 -0400
起就有特定于Ubuntu的版本Ubuntu特定版本的完整列表在这里：
https://launchpad.net/ubuntu/xenial/+source/expat
我该怎么做才能使pbuilder重新工作？
P.S。我指定使用DigitalOcean是因为apt默认使用它们的镜像。但是，看来pbuilder使用的是普通列表：
root@build:/# cat /etc/apt/sources.list
deb http://archive.ubuntu.com/ubuntu xenial-proposed main restricted universe multiverse
deb http://archive.ubuntu.com/ubuntu xenial main restricted universe multiverse
#deb-src http://archive.ubuntu.com/ubuntu xenial main restricted universe multiverse
直到2018年10月17日，它工作了好几年……
更新：
以防万一，我尝试创建pbuilder/xenial-base.tgz的新版本。命令：
rm -rf pbuilder
pbuilder-dist xenial create
...
The following packages have unmet dependencies:
build-essential : Depends: gcc (>= 4:5.2) but it is not going to be installed
Depends: g++ (>= 4:5.2) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
...
我们可以看到，即使只是尝试在Ubuntu 16.04上创建新的构建环境，现在似乎也失败了。结果我posted a bug。
更新2
我现在在另一台计算机上进行了测试，该计算机是DigitalOcean计算机，并且出现了完全相同的错误。再次重申，我确定Ubuntu的源软件包中存在问题。哦，好吧！