build system improvements

It includes:
- merge package-builder's and SpecDeps's spec parser to avoid code
  duplication
- spec parser: support for Requires(post|pre|postun|preun)
- spec parser: support for conditional requires, such as
  "Requires: (aaa <= 3.1 or bbb) ccc (ddd or fff > 4.5.6)"
- add "file to owner package" mapping. "/sbin/useradd -> shadow" to
  install proper build requires
- create SPECS singleton to store parsed spec data
- use constants.py only to store constants

Change-Id: I9fbb9ccdb9fdfef932d4097983e02eb41bb73987
Reviewed-on: http://photon-jenkins.eng.vmware.com:8082/3819
Tested-by: gerrit-photon <[email protected]>
Reviewed-by: Vinay Kulkarni <[email protected]>
Reviewed-by: Xiaolin Li <[email protected]>