Remove dependency from Host to python

Summary:
The only reason python was used in the Host module was to compute the
python path. I resolve this the same way as D47384 did for clang, by
moving the path computation into the python plugin and modifying
SBHostOS class to call into this module for ePathTypePythonDir.

Reviewers: zturner, jingham, davide

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D48215

llvm-svn: 335104