fix #1556: dub --cache=local fetch + build

Building a package that was locally cached failed, because
the dependencies whould be fetched at the wrong destination.

For example if one tries to build the package 'optional':

$ dub --cache=local fetch optional
-> optional is now at ./.dub/package/optional
$ dub --cache=local build optional
-> fails because the dependency bolts was fetched to
./.dub/package/optional/.dub/package/bolts.

This is fixed by not overwriting the root path if dub builds
a package from the cache.

source/dub/commandline.d

@@ -714,7 +714,6 @@ abstract class PackageBuildCommand : Command {
 		auto pack = dub.packageManager.getFirstPackage(package_name);
 		enforce(pack, "Failed to find a package named '"~package_name~"' locally.");
 		logInfo("Building package %s in %s", pack.name, pack.path.toNativeString());
-		dub.rootPath = pack.path;
 		dub.loadPackage(pack);
 		return true;
 	}

test/issue1556-fetch-and-build.sh

@@ -0,0 +1,15 @@
+#!/usr/bin/env bash
+DIR=$(dirname "${BASH_SOURCE[0]}")
+
+. "$DIR"/common.sh
+
+dub remove main-package --non-interactive --version=* 2>/dev/null || true
+dub remove dependency-package --non-interactive --version=* 2>/dev/null || true
+
+
+echo "Trying to fetch fs-sdl-dubpackage"
+"$DUB" --cache=local fetch main-package --skip-registry=all --registry=file://"$DIR"/issue1556-fetch-and-build-pkgs
+
+echo "Trying to build it (should fetch dependency-package)"
+"$DUB" --cache=local build main-package --skip-registry=all --registry=file://"$DIR"/issue1556-fetch-and-build-pkgs
+