Do not skip the root path if it's a dotdir

Do not skip the root path if it's a dotdir

When traversing the directory tree, cargo will omit paths that start
with a period character to avoid interfering with other software.
Unfortunately this also prevents a crate from being located in a
directory prefixed with a period. Address this by extending the test
against the traversal root that already guards Git submodules.

Fixes issue #1999 which was introduced with commit 11144645f..