Cross compiling for xenial always fails (E: Unable to locate package ...)

This bug is driving me crazy. And it's not limited to xenial either - same problem with other distros.

There must be something special about those packages that can't be located. Are they new-ish or different in any way?

Fix committed into lp:mir at revision None, scheduled for release in mir, milestone 0.25.0

This bug was fixed in the package mir - 0.25.0+17.04.20161203-0ubuntu1

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mir (0.25.0+17.04.20161203-0ubuntu1) zesty; urgency=medium

  * New upstream release 0.25.0 (https://launchpad.net/mir/+milestone/0.25.0)
    - ABI summary:
      . mirclient ABI unchanged at 9
      . mirserver ABI bumped to 42
      . mircommon ABI bumped to 7
      . mirplatform ABI bumped to 14
      . mirprotobuf ABI unchanged at 3
      . mirplatformgraphics ABI bumped to 11
      . mirclientplatform ABI unchanged at 5
      . mirinputplatform ABI bumped to 6
      . mircore ABI added, at version 1
    - Enhancements:
      . Added pointer confinement support
      . Identify Raspberry Pi output types correctly
      . Added dead key and compose key support
      . Added gamma support for KMS hardware
      . Added yakkety and zesty gcc-6 support
      . Added surface passthrough support for fullscreen clients of nested
        servers
      . Introduced a new library `libmircore'
      . Began migrating from Protobuf to Cap'n Proto starting with events
      . Began support for VirtualBox (servers now start instead of crashing
        but full support for GL clients is still missing)
      . Began support for high precision frame timing, implemented for the
        KMS, Android and X11 backends so far
      . Added --display-report support for the X11 backend
      . Improved reliability of `mirvanity' latency test tool
      . Moved to using the CSS cursor naming scheme internally, although
        the old symbolic indentifiers remain unchanged and still work
      . Added refresh rate to MirSurfaceOutputEvent
      . Many improvements to automated test quality and coverage
      . Smoother desktop zoom using mir_proving_server (Super + two fingers)
    - Bugs fixed:
      . FTBFS On Zesty: invalid use of
        'struct native_handle' (LP: #1638774)
      . libmirclient-debug-extension-dev install debug/surface.h
        twice (LP: #1639153)
      . Mir crashes on virtual box drmModeCrtcSetGamma
        Function not implemented (LP: #1638220)
      . Flickering in mir_demo_client_multistream (LP: #1635273)
      . Camera apps (camera & vanity) never exit (LP: #1635010)
      . Clients cannot connect when MIR_SOCKET="" (LP: #1634508)
      . mirtest-dev packaging fails to sepcify boost depends (LP: #1633537)
      . cursor.h is unclear about mir_diagonal_resize_* (LP: #1626924)
      . Inconsistent underscores in cursor names (LP: #1632617)
      . [regression] Accidental libmircommon.so.6 break (LP: #1617865)
      . [regression] mir demo server fails to start with mx4 (LP: #1615703)
      . [regression] Trust prompt now shown when starting camera (LP: #1612256)
      . [regression] make abi-check fails to build (LP: #1626866)
      . Nested servers prevent overlays or fullscreen bypass (LP: #1262116)
      . Failed to get PRIME fd from gbm bo (LP: #1607812)
      . Mir-on-X is difficult to use on 1366x768 laptop (LP: #1590959)
      . Remove useless message 'Surface 0 DPI' in demos (LP: #1559831)
      . Mir fails to cross-build with newer sbuild (LP: #1628507)
      . Infinite loop in AbstractShell::focus_next_session (LP: #1625401)
      . Dest...