Update tex builder to handle the case where a \input{foo}

Update tex builder to handle the case where a \input{foo}
command tries to work with a directory named foo instead of the
file foo.tex. The builder now ignores a directory and continues
searching to find the correct file.

git-svn-id: http://scons.tigris.org/svn/scons/trunk@4642 fdb21ef1-2011-0410-befe-b5e4ea1792b1