Computes transitive exclusive maps.

  This computes transitive exclusive tags for a dependency graph rooted
  with a set of build targets specified by a user. If this process produces
  any collisions where a single target contains multiple tag values for a single
  exclusives key, then it generates an error and the compilation will fail.

  The syntax of the exclusives attribute is: ::

     exclusives = {"id": "value", ...}

  For example, suppose that we had two java targets, jliba and jlibb. jliba uses
  slf4j, which includes in its jar package an implementation of log4j. jlibb uses
  log4j directly. But the version of log4j that's packaged inside of slf4j is
  different from the version used by jlibb. ::

     java_library(name='jliba',
       depedencies = ['slf4j-with-log4j-2.4'])
     java_library(name='jlibb',
       dependencies=['log4j-1.9'])
     java_binary(name='javabin', dependencies=[':jliba', ':jlibb'])

  In this case, the binary target 'javabin' depends on both slf4j with its
  packaged log4j version 2.4, and on log4j-1.9.
  Pants doesn't know that the slf4j and log4j jar_dependencies contain
  incompatible versions of the same library, and so it can't detect the error.

  With exclusives, the jar_library target for the joda libraries would declare
  exclusives tags: ::

     jar_library(name='slf4j-with-log4j-2.4', exclusives={'log4j': '2.4'})
     jar_library(name='joda-2.1', exclusives={'log4j': '1.9'})

  With the exclusives declared, pants can recognize that 'javabin' has conflicting
  dependencies, and can generate an appropriate error message.

  Data about exclusives is provided to other tasks via data build products.
  If the build data product 'exclusives_groups' is required, then an
  ExclusivesMapping object will be created.