externalsrc: fix lookup for .gitmodules

Commit 0533edac277080e1bd130c14df0cbac61ba01a0c broke
bitbake parsing when bitbake is executed from directory with existing .gitmodules
and the recipe in externalsrc does not have .gitmodules

The check needs to search for .gitmodules in sources path, not cwd.

iParsing recipes...ERROR: ExpansionError during parsing <path to recipe>
...
bb.data_smart.ExpansionError: Failure expanding variable do_compile[file-checksums], expression was ${@srctree_hash_files(d)} which triggered exception CalledProcessError: Command '['git', 'config', '--file', '.gitmodules', '--get-regexp', 'path']' returned non-zero exit status 1.

Signed-off-by: Peter Marko <peter.marko@siemens.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
(cherry picked from commit 66ff3d1f)
Signed-off-by: Steve Sakoman <steve@sakoman.com>