Allow user to manually choose sources directory during conversion

[rmartin16]

What is the problem or limitation you are having?

The briefcase convert command should allow the user to choose a sources directory if a suitable one cannot be found instead of raising. For instance, if my project was named "My Favorite App", Briefcase is expecting a directory named "my_favorite_app"...but "myfavoriteapp" is reasonable and should be usable by the convert command.

Describe the solution you'd like

Allow users to choose an arbitrary sub-directory for the app sources.

Describe alternatives you've considered

Re-organize the existing project prior to conversion.

Additional context

No response

[freakboy3742]

Agreed that it shouldn't be raising an error... that's an oversight of review on my part. The rest of the infrastructure is there to prompt the user - but we're raising an error before we get to that logic. The only complication I can see is that it's not obvious what the default value should be - but maybe a blank string will work (because it will be rejected by validation).