Add a PreGenerateBuildActions pass

Call PreGenerateBuildActions on each module that implments it after
resolving dependencies but before calling GenerateBuildActions on
any module.  This allows the build logic to propagate top-down
information, for example what variants of a library need to be
compiled for the binaries it is used in.

Change-Id: I2737504fa9d1a2b42ef747497de32c1c5129233d