Add support for local package installation with pkgng.

[amishHammer]

This patch allows local packages to be installed if available.

[TheDreamer]

Just what I've been looking for since I switched to pkgng.... because I use '-g' and '--delete-build-only', its nice having '--packages-build --packages-local --local-packagedir=...', to install the build only dependency from previous package.

Don't know if it always did, and I only noticed now in testing this patch... but:

 ===>>> Installing package
 Installing libtool-2.4.2_2... done

 ===>>> Creating a package for new version libtool-2.4.2_2
 Creating package for libtool-2.4.2_2
   ===>>> Package saved to /usr/ports/packages/All

 ===>>> Installation of devel/libtool (libtool-2.4.2_2) succeeded

creating a package from an installing a package seems strange.

[TheDreamer]

Since pkgng has the feature...

when installing a build-only depend..."pkg add" should use the "-A" switch.

Not sure if it should with other automatic dependency installs by portmaster?