Reproducible BUILD_PATH_PREFIX_MAP

Make builds reproducible by honoring BUILD_PATH_PREFIX_MAP
Upstream has rejected the patch in this form and promised to implement an
alternative they are happy with instead. That hasn't happened yet though.

Bug: https://github.com/golang/go/issues/22491, https://github.com/golang/go/issues/16860
Forwarded: https://golang.org/cl/73291 (rejected upstream though)

Gbp-Pq: Name 0001-Reproducible-BUILD_PATH_PREFIX_MAP.patch

golang-1.12 (1.12.7-3+rpi1) bullseye-staging; urgency=medium

  [changes brought forward from golang-1.10 1.10~rc2-1+rpi1 by Peter Michael Green <plugwash@raspbian.org> at Sat, 24 Feb 2018 12:22:04 +0000]
  * Build with GOARM=6
  * Disable testsuite.
  * Fix clean target.

  [changes introduced in golang-1.12 1.12.7-3+rpi1 by Peter Michael Green]
  * Further clean target fixing.

[dgit import unpatched golang-1.12 1.12.7-3+rpi1]

Import golang-1.12_1.12.7-3+rpi1.debian.tar.xz

[dgit import tarball golang-1.12 1.12.7-3+rpi1 golang-1.12_1.12.7-3+rpi1.debian.tar.xz]

Import golang-1.12_1.12.7.orig.tar.gz

[dgit import orig golang-1.12_1.12.7.orig.tar.gz]