Imports fail if there's also a directory of the same name

[QuLogic]

As in Compass, say you have a partial and a directory with a similar name. On Linux (and likely on any Unix, where "everything is a file"), if you import only the basename of the partial (i.e., without the extension), then libsass will open the directory instead of the partial and fail to read anything.

Test case:

$ touch bar.scss
$ mkdir bar
$ cat > foo.scss << EOF
@import "bar";
EOF
$ sassc foo
ERROR -- foo.scss, line 1: error reading file "bar"

[akhleung]

Merged the pull request; thanks for the fix!