[vpython] Do not load `.vpython` directories as specs.
[vpython] Do not load `.vpython` directories as specs.

We used to use `~/.vpython` on developer machines as the vpython root,
so vpython might find this directory and decide that it's a spec file.

Use `!IsDir()` instead of `Mode().IsRegular()` to still allow for
symlinks.

R=dnj@chromium.org, 9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org

Bug:
Change-Id: I16ce2b4b2c9072cbf83aad89b277f2dbea13e34b
Reviewed-on: https://chromium-review.googlesource.com/851263
Reviewed-by: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>
Commit-Queue: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go