add expected-files check, rename '--unexpected-file-patterns' to '--expected-files' (fixes #233)

[jameslamb]

Fixes #233.

Adds a new check, [expected-files], which allows the use of pydistcheck to assert that a distribution contains certain files or directories.

For example:

[tool.pydistcheck]
expected_files = [
    # at least one Python source file
    "*.py",
    # 0 static libraries
    "!.a",
    # exactly this specific Jinja2 template
    "coolpkg/resources/templates/the-template.j2"
]

Notes for Reviewers

Calling this breaking because it changes some parameters:

• --unexpected-directory-patterns to --expected-directories
• --unexpected-file-patterns to --expected-files