The APEX dependency is more correctly tracked. Previously, the
dependency was tracked while we gather modules that will be installed to
an APEX. This actually was incorrect because we skipped many dependency
types that we don't need to follow to gather the modules list, such as
the headers dependency.
Now, the dependency is tracked directly when a module is mutated for an
APEX. In other words, if a module is mutated for an apex X, then the
module will appear in the X-deps-into.txt file.
This change also changes the format of the txt file. It now clearly
shows why a module is included in the APEX by showing the list of
modules that depend on the module.
Bug: 146323213
Test: m
Change-Id: I0a70cf9cce56e36565f9d55683fdaace8748a081
678c881a4f