* Rename `platform_wordsize_qualify` to `find_expected_file`, and make
  it return a filename instead of an (absolute) filepath.

* Replace most usages of `qualify` by `in_testdir`. Others usage sites
  will be deleted in a later commit.

These changes will be useful in a later commit, when we'll distinguish
between files in the source directory and those in a (newly created)
test directory.

Reviewed by: austin, bgamari

Differential Revision: https://phabricator.haskell.org/D1186