libxbps: separate virtualpkg conf stuff from repository_pool_find_pkg.

Now to find a virtualpkg set by the user in conf file, there is a new
function: xbps_repository_pool_find_virtualpkg(). Use it in some
places to make jpeg -> libjpeg-turbo (and other pkgs that can be
replaced with other virtual pkgs) work correctly even if installed
multiple times.

lib/repository_findpkg.c

@@ -87,10 +87,14 @@ repository_find_pkg(const char *pattern, const char *reason)
 	pkg_repod = xbps_repository_pool_find_pkg(pattern,
 	    bypattern, bestpkg);
 	if (pkg_repod == NULL) {
-		/* not found */
-		rv = errno;
-		errno = 0;
-		goto out;
+		pkg_repod = xbps_repository_pool_find_virtualpkg(pattern,
+		    bypattern, bestpkg);
+		if (pkg_repod == NULL) {
+			/* not found */
+			rv = errno;
+			errno = 0;
+			goto out;
+		}
 	}
 	/*
 	 * Prepare transaction dictionary and missing deps array.