fix dlang#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.sh

@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+
+. $(dirname "${BASH_SOURCE[0]}")/common.sh
+
+${DUB} --cache=local remove optional || true
+${DUB} --cache=local remove bolts || true
+${DUB} --cache=local fetch optional
+${DUB} --cache=local build optional --compiler=${DC}
+