Add/omit the HeadersPath Info.plist key for XCFramework with static libraries.

[keith]

An XCFramework with static libraries can include public Objective-C(++) headers using the public_hdrs rule attribute, or generated headers from the underlying Swift module.

This change updates the XCFramework Info.plist generation logic to add or omit the HeadersPath key if there are any header files.

PiperOrigin-RevId: 499574821 (cherry picked from commit e00404ac6a4d1949674e6ed9e47d456660cd5d26)

[keith]

the actual change here wasn't relevant because we produce frameworks not libraries for this rule, so I'm not sure if we'll take these tests or not