Force 15.0->Current symlinks to exist and function

NuGet Restore on sln files fails unless /usr/bin/msbuild references
15.0, so we need that symlink to exist. Force the 15.0 directory to
go away, so we can use the link from the package.

debian/msbuild.install

@@ -1,5 +1,7 @@
 /usr/bin/
-/usr/lib/mono/xbuild/
+/usr/lib/mono/xbuild/Microsoft
+/usr/lib/mono/xbuild/Current
+/usr/lib/mono/xbuild/*.dll*
 /usr/lib/mono/msbuild/Current/bin/Sdks/
 /usr/lib/mono/msbuild/Current/bin/SdkResolvers/Microsoft.Build.NuGetSdkResolver
 /usr/lib/mono/msbuild/Current/bin/*.config

debian/msbuild.links

@@ -1 +1,2 @@
 /usr/lib/mono/msbuild/Current	/usr/lib/mono/msbuild/15.0
+/usr/lib/mono/xbuild/Current	/usr/lib/mono/xbuild/15.0

debian/msbuild.preinst

@@ -0,0 +1,20 @@
+#!/bin/sh -e
+
+OLD_XBUILDDIR=/usr/lib/mono/xbuild/15.0
+OLD_MSBUILDDIR=/usr/lib/mono/msbuild/15.0
+
+if [ "$1" = "upgrade" -o "$1" = "install" ] ; then
+   if [ -d "$OLD_XBUILDDIR" -a ! -L "$OLD_XBUILDDIR" ] ; then
+     echo "Found old XBuild tools directory, moving out of the way"
+     mv $OLD_XBUILDDIR $OLD_XBUILDDIR.old
+   fi
+fi
+
+if [ "$1" = "upgrade" -o "$1" = "install" ] ; then
+   if [ -d "$OLD_MSBUILDDIR" -a ! -L "$OLD_MSBUILDDIR" ] ; then
+     echo "Found old MSBuild tools directory, moving out of the way"
+     mv $OLD_MSBUILDDIR $OLD_MSBUILDDIR.old
+   fi
+fi
+
+#DEBHELPER#

debian/rules

@@ -15,7 +15,6 @@ override_dh_auto_build:
 override_dh_auto_install:
 	HOME=$(TEMP_HOME) ./artifacts/mono-msbuild/msbuild mono/build/install.proj /p:MonoInstallPrefix=$(DEBIAN_DIR_ABS)/tmp/usr /p:Configuration=Release-MONO /p:IgnoreDiffFailure=true
 	sed -i "s@$(DEBIAN_DIR_ABS)/tmp@@g" debian/tmp/usr/bin/msbuild
-	sed -i "s@15.0@Current@g" debian/tmp/usr/bin/msbuild
 	find debian/tmp/usr/lib/mono/ -name Microsoft.DiaSymReader.Native.*dll -delete
 	find debian/tmp/usr/lib/mono/ -name *.dylib -delete