Merging r343347:

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r343347 | cmatthews | 2018-09-28 10:55:18 -0700 (Fri, 28 Sep 2018) | 4 lines

make lit builtins a package

cat.py is not being installed when lit is installed from source. So
tests that use the internal shell fail when using cat.
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llvm-svn: 344792