Currently the PKG_CONFIG_PATH environmental variable is set to `$SNAPCRAFT_STAGE/lib/pkgconfig` during build step, however, some parts install their pc files at `$SNAPCRAFT_STAGE/lib/$DEBIAN_MULTIARCH_TUPLE/pkgconfig` and as a result the files are not read during the building of other parts. IMO `$SNAPCRAFT_STAGE/lib/$DEBIAN_MULTIARCH_TUPLE/pkgconfig` should also be set as a fallback path.