xbps-src: multiple performance improvements.

- use xbps-checkvers(1) to resolve dependencies.
- all dependencies are installed at once for the host and target.
- the `show-build-deps` target is now much faster.
- the `update-bulk/show-repo-updates` targets are now much faster.
- the `update-sys/show-sys-updates` targets are now much faster.
- the `bootstrap` target now works on musl hosts.
- simplified some loops.
- use cut(1) rather than awk(1) where applicable.
- multiple random changes to improve performance.

Based on work started by @Duncaen on https://github.com/void-linux/void-packages/pull/12433

Close https://github.com/void-linux/void-packages/pull/12433
Close https://github.com/void-linux/void-packages/pull/11282

common/hooks/do-pkg/00-gen-pkg.sh

@@ -32,16 +32,16 @@ genpkg() {
 
 	_preserve=${preserve:+-p}
 	if [ -s ${PKGDESTDIR}/rdeps ]; then
-		_deps="$(cat ${PKGDESTDIR}/rdeps)"
+		_deps="$(<${PKGDESTDIR}/rdeps)"
 	fi
 	if [ -s ${PKGDESTDIR}/shlib-provides ]; then
-		_shprovides="$(cat ${PKGDESTDIR}/shlib-provides)"
+		_shprovides="$(<${PKGDESTDIR}/shlib-provides)"
 	fi
 	if [ -s ${PKGDESTDIR}/shlib-requires ]; then
-		_shrequires="$(cat ${PKGDESTDIR}/shlib-requires)"
+		_shrequires="$(<${PKGDESTDIR}/shlib-requires)"
 	fi
 	if [ -s ${XBPS_STATEDIR}/gitrev ]; then
-		_gitrevs="$(cat ${XBPS_STATEDIR}/gitrev)"
+		_gitrevs="$(<${XBPS_STATEDIR}/gitrev)"
 	fi
 
 	# Stripping whitespaces