Bug#872745: dh_install: now bails for missing files in disabled packages

[Helmut Grohne <helmut@subdivi.de>]

Package: debhelper
Version: 10.7.2
File: /usr/bin/dh_install
User: helmutg@debian.org
Usertags: rebootstrap
Control: affects -1 + src:gnumach

I noticed that cross building a gnumach stage1 for hurd-i386 started to
fail. Sucessful log:

https://jenkins.debian.net/job/rebootstrap_hurd-i386_gcc7/7/console

Failing log:

https://jenkins.debian.net/job/rebootstrap_hurd-i386_gcc7/9/console

Interesting bit: Both use gnumach 2:1.8+git20170609-1. Notable
difference: debhelper 10.2.5 vs 10.7.2.

The failing command is:

| dh_install -pgnumach-image-1.8-486 --sourcedir=/tmp/buildd/gnumach_1/gnumach-1.8+git20170609/debian/tmp boot

In this command line, gnumach-image-1.8-486 is a package disabled by the
profile stage1 and stage1 is active. The file "boot" isn't built an
unavailable. (Applies to both logs.) It seems that gnumach assumes that
"dh_install -pfoo ..." is a noop (regardless of the files referenced) if
foo is disabled by a profile. That assumption was broken somewhere from
debhelper 10.2.5 to 10.7.2.

Whether that assumption is a good one is beyond my judgement. Imo, the
documentation does neither contradict nor support it.  If that
assumption is reasonable, it should be documented and restored in
debhelper. Otherwise the bug is found in gnumach and should be fixed
there. Please choose the victim reasonably.

Helmut