[BuildStream] Invalid characters for element names

[Chandan Singh <chandan chandansingh net>]

Hi all,

As we already use colon (`:`) as a separator for specifying cross-junction
elements, I was wondering if it would make sense to disallow elements to have
it in their names. This was bought up in comments on #809 [1] and !998 [2].

As we discussed on GitLab, this is already an issue for the CLI as it will not
work correctly if a project has a junction element with `:` in its name. So, I
think we should maintain a list of invalid characters and disallow elements
that violate this policy. I imagine this check to work in the same the recent
change for mandating `.bst` extension works.

While we are doing this change, are there any characters that we would like to
disallow from element names, and reserve them for internal use?

If I don't hear back on this, I will assume that there are no objections and
will proceed to submitting a merge request for the change.

[1]: https://gitlab.com/BuildStream/buildstream/issues/809#note_123194269
[2]: https://gitlab.com/BuildStream/buildstream/merge_requests/998#note_123204414

Thanks,
Chandan