[build2] bdep new allows projects names that are invalid c++ namespace names

[Boris Kolpackov]

Klaim - Joël Lamotte <mjklaim at gmail.com> writes:

> Is this fix only for C++ or for any language?

Currently the only supported by bdep-new(1) languages are C and C++ which
have the same identifier rules. But the implementation is structured in
such a way that if support for another language is added, then its
identifier rules will be added naturally:

https://github.com/build2/bdep/blob/master/bdep/new.cxx#L154-L170