Pass pointer to dependencyTag instead of copying struct

A follow up change will add some more fields to the tag which will make
passing the struct around by value will get more expensive. Switching
to pointers will make this slightly more efficient.

Changing the type of the tags from dependencyTag to *dependencyTag
broke a test. Rather than simply patch the test and then maybe have to
patch it again in the next change this adds a a String() method that
will insulate the test from being affected by changes in the
dependencyTag contents.

Bug: 232401814
Test: m nothing
Change-Id: I23da742ebffb74ef3b9b68f772519ceb38332f5f