Skylint: improve the naming conventions checker

Changes in behavior:
 * Local variables are also allowed to be UPPER_SNAKE_CASE.
   Upper case means that they're constants but this is not checked yet.
 * Providers are required to be UpperCamelCase. A variable FooBar
   is considered a provider if it appears in an assignment of the form
   "FooBar = provider(...)"
 * Shadowing of builtins (e.g. "True = False", "def fail()") is
   not allowed
 * The single-letter variable names 'O', 'l', 'I' are disallowed
   since they're easy to confuse
 * Multi-underscore names ('__', '___', etc.) are disallowed
 * Single-underscore names may only be written to, as in
     a, _ = tuple
   They may not be read, as in "f(_)".

In the process, I also moved some code from UsageChecker to
AstVisitorWithNameResolution to prevent duplication in
NamingConventionsChecker.

RELNOTES: none
PiperOrigin-RevId: 168250396

0 files changed, 0 insertions, 0 deletions