Bugfix false positive for "unused" variable for nested defs

[oreflow]

When detecting whether symbols are used or not, variables referenced as argument defaults of the inner def would count as "use" in the inner scope, and hence falsely trigger "unused" warning.

Example:

def outer_def(name, foo = "foo"):
    def inner_def(foo = foo):
        print(foo)  # buildifier: disable=print

    inner_def()

Unused variable check for the above function would

1. Check outer_def for variable usage
2. Recursively check inner_def for variable usage

• Detect that foo is used
• Omit foo from returned usedSymbolsFromOuterScope since foo is a local variable

3. Receive no "usedSymbol" for foo
4. Output "unused-variable" warning

If there are RHS idents in a def statement, these idents are always from an outer scope, so these can check the later inner-scope definedSymbols check.