Add support for nested requirements files

pip supports requirements files with a line such as:

-r other-requirements.txt

for specifying nested requirements files.  Adding support to pbr
should be helpful with handling optional dependencies in projects
better, by allowing us to split the requirements files and then
merge them back together using -r lines.

Change-Id: I80e081b2229cc81eed26533c50afb07dc98a1db1
Merge "Add support for nested requirements files"