dib-run-parts should dereference symlinks

dib-run-parts should dereference symlinks, and if they point to an
executable file, execute that file as a script. This can be accomplished
by using the xtype predicate in the find command instead of the type
predicate.

This change is needed so that we can dynamically symlink hook scripts
into hook directories such as install.d at runtime to support different
install types.

Change-Id: I933e7f4b4dcf16956841d8c14aa63b0f9a18fc5d
Symlink correct element install type

The source-repositories element will now symlink the install scripts
corresponding to the set install type for an element into the install.d
directory.

Different install types are implemented by elements by writing scripts to do
the install type under install.d/<element-name>-<install-type>-install
directories.

For example, the nova element would provide:

nova/install.d/nova-package-install/74-nova
nova/install.d/nova-source-install/74-nova

source-repositories will create the following symlink for the package
install type:

$TMP_HOOKS_PATH/install.d/74-nova -> nova-package-install/74-nova

Or, for the source install type:

$TMP_HOOKS_PATH/install.d/74-nova -> nova-source-install/74-nova

Change-Id: I1bfaf39e5a98b2af904fbc6d674dbada30b27ed9
Merge "dib-run-parts should dereference symlinks"
Merge "Symlink correct element install type"