add ignore marker file to default project searcher

[mbway]

Currently, the DefaultProjectFileSearcher is very conservative, but I think some changes it might provide a better experience for the majority of users.

I ran into a case where I had a package with a tests/ directory (containing only .py files) and I realised there was no built-in way to ignore this directory other than passing a custom ProjectFileSearcher to install() which is not ideal when working in a team.

With this change, a .maturin_hook_ignore file could be added to tests/ to have that directory ignored.

Additionally, I don't think there are many use cases for depending on .py files for the compiled part of a package, but it is very common to have .py files in the repository of a maturin package that do not need to trigger a rebuild.
If a package depends on .py files at compile time with include_str!() or similar then a custom ProjectFileSearcher can be passed, or the following can be put before calling install():

maturin_import_hook.project_importer.DefaultProjectFileSearcher.DEFAULT_SOURCE_EXCLUDED_FILE_EXTENSIONS.discard('.py')