Allow wildcards in `sources` list

As shown in https://github.com/beeware/briefcase/discussions/1117, the need to list every subdirectory of `src` individually is easy to miss, even for me. If the `sources` setting accepted wildcards, then the default in the new project template could simply be `src/*`, and most projects would never need to change it.

Start by reading the `sources` setting implementation and the new project template, then review discussion #1117 for the expected wildcard behavior. Confirm how source subdirectories are currently collected and identify the relevant tests; the work is done when a default such as `src/*` covers the intended subdirectories without requiring each one to be listed individually.

Written by the indexing model from the issue text.