BPathFinder: Fix resolvable expression support

We need to use the package's .self symlink, since that does point to
the package's installation location. The containing directory lives in
/boot/system/package-links and would therefore always resolve to the
system installation location.

BPathFinder: Add BPackageResolvableExpression initialization

Add a constructor and a SetTo() method with a
BPackageResolvableExpression parameter instead of a path. The path of
the package satisfying the expression is used.

The new functionality lives in libpackage as it uses the package kit.

BPathFinder: Fix resolvable expression support

We need to use the package's .self symlink, since that does point to
the package's installation location. The containing directory lives in
/boot/system/package-links and would therefore always resolve to the
system installation location.

BPathFinder: Add BPackageResolvableExpression initialization

Add a constructor and a SetTo() method with a
BPackageResolvableExpression parameter instead of a path. The path of
the package satisfying the expression is used.

The new functionality lives in libpackage as it uses the package kit.