[BuildStream] Changing element files extension requirements

[Phillip Smyth <phillip smyth codethink co uk>]

Hi all,

Following on from a discussion on #buildstream on IRC, there have been 
suggestions of making file extension like `*.bst` mandatory.

This would currently be of use with the introduction of build all and 
show all commands, as currently it is difficult to know if a file is an 
element or some other kind of file, and the current proposed solution 
isn't fool-proof as it assumes that only element files will exist inside 
of the element-path directory, which may not be the case, and in some 
instances, the element-path directory does not exist.

One suggestion was to introduce a "element-extensions:" config which 
could be used to specify elements, however there is the issue of a 
default value being required, and leaving the default as blank or "*", 
would result in this providing no guarantees of consistency.

I'd appreciate everyone's input on this matter as it is quite a major 
change.

Kind regards,

Phillip Smyth (Nexus)