Make get_base_path() and get_dir_name() work in include context

Summary: Closes issue #200.

get_base_path() works only in build file context. Make it works also
in include context.

Introduce new method: get_dir_name() to retrieve dirname for the current
build or include file.

Test Plan:
* Add buildfile.includes = //bucklets.defs line to .buckconfig
* Put this line: d = get_dir_name() in bucklets.defs
* Verify that the root directory of project is returned