tests: consistently use pyflakes as a Python module

We check availability of pyflakes as a module, and also running it for real as
a module. Only fair to test filterpyflakes.py working correctly when using
pyflakes as a module too.

This is a graft of a similar fix that ended up on default.

Differential Revision: https://phab.mercurial-scm.org/D8629

tests/test-check-pyflakes.t

@@ -1,26 +1,26 b''
 #require test-repo pyflakes hg10
 
   $ . "$TESTDIR/helpers-testrepo.sh"
 
 run pyflakes on all tracked files ending in .py or without a file ending
 (skipping binary file random-seed)
 
   $ cat > test.py <<EOF
   > print(undefinedname)
   > EOF
-  $ pyflakes test.py 2>/dev/null | "$TESTDIR/filterpyflakes.py"
+  $ $PYTHON -m pyflakes test.py 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   test.py:1: undefined name 'undefinedname'
   
   $ cd "`dirname "$TESTDIR"`"
 
   $ testrepohg locate 'set:**.py or grep("^#!.*python")' \
   > -X hgext/fsmonitor/pywatchman \
   > -X mercurial/pycompat.py -X contrib/python-zstandard \
   > -X mercurial/thirdparty \
   > 2>/dev/null \
   > | xargs $PYTHON -m pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
   contrib/perf.py:*: undefined name 'xrange' (glob) (?)
   mercurial/hgweb/server.py:*: undefined name 'reload' (glob) (?)
   mercurial/util.py:*: undefined name 'file' (glob) (?)
   mercurial/encoding.py:*: undefined name 'localstr' (glob) (?)