build: build deb/rpm independently on config/extensions in the host system

Reverts 5aac617a028d and replaces it with a more general solution.

- works for both rpm and deb
- sidesteps eventual problems with local extensions that have nothing to do with
  the build process (hg-git, for example, fails with version 4.4 because
  dedab036215d removed peerrepository, and hg-git still uses it as of 0.8.9)

contrib/buildrpm

@@ -11,7 +11,6 b''
 
 BUILD=1
 RPMBUILDDIR="$PWD/rpmbuild"
-export HGPLAIN=
 
 while [ "$1" ]; do
     case "$1" in

contrib/packagelib.sh

@@ -8,6 +8,9 b''
 #
 # node: the node|short hg was built from, or empty if built from a tag
 gethgversion() {
+    export HGRCPATH=
+    export HGPLAIN=
+
     make cleanbutpackages
     make local PURE=--pure
     HG="$PWD/hg"