windows: define normcase spec and fallback
These will be used in upcoming patches to efficiently create a dirstate
foldmap.
$ echo 'raise Exception("bit bucket overflow")' > badext.py
$ abspath=`pwd`/badext.py
$ cat <<EOF >> $HGRCPATH
> [extensions]
> gpg =
> hgext.gpg =
> badext = $abspath
> badext2 =
> EOF
$ hg -q help help
*** failed to import extension badext from $TESTTMP/badext.py: bit bucket overflow
*** failed to import extension badext2: No module named badext2
hg help [-ec] [TOPIC]
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