npm: accept unspecified versions in package.json

npm: accept unspecified versions in package.json

Our current emulation mandates that the package.json contains a version
field. Some packages may not provide it when they are not published to
the registry. The actual `npm pack` would allow such packages, so
should we.

This patch adds a default value to allow building such packages.

This applies for instance to this package which doesn't declare a
version:
 - https://github.com/cockpit-project/cockpit/blob/23701a555a5af13f998ee4c7526d27fdb5669d63/package.json#L2

v3:
 - Split bitbake npmsw.py modification in another commit

Co-authored-by: Tanguy Raufflet <tanguy.raufflet@savoirfairelinux.com>
Signed-off-by: Tanguy Raufflet <tanguy.raufflet@savoirfairelinux.com>
Signed-off-by: Enguerrand de Ribaucourt <enguerrand.de-ribaucourt@savoirfairelinux.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>