Consider aligning file_names lint with unquoted imports proposal

[munificent]

The language team is working on a proposal to allow unquoted imports (and exports and probably parts) like:

import foo/bar.baz;

Since the import path isn't quoted, some file or directory names don't work if the lexer gets confused and thinks they are number literals. Also, the proposal specifically supports path components that are multiple dotted components like foo.bar.baz (largely because that's common inside google3).

As I understand it, the file_names lint currently only applies to the library's file name and not any directory names above it. Also, it (I think?) mostly ignores file paths that contain . (aside from the .dart). And I think it doesn't fire if a path name is only digits.

If that proposal goes forward (which is likely but not certain), it might be good to align the lint with the proposal's restrictions so that users can start fixing their file names before the feature ships.