Fix Literal parse error in RemoveImportsVisitor

[camillol]

GatherNamesFromStringAnnotationsVisitor, which is called by GatherUnusedImportsVisitor, which is called by RemoveImportsVisitor, attempts to parse any string contained in an annotation as if it were Python code. This behavior was added in #353 with the goal of parsing type names wrapped in strings.

However, not all strings occurring inside annotations can or should be interpreted that way. For example:

from typing import Literal

a: Literal["in"]
b: list[Literal["1x"]]
c: Literal["Any"]

a and b will cause a ParserSyntaxError, while c will be incorrectly treated as a mention of the symbol Any.

This PR makes two changes to GatherNamesFromStringAnnotationsVisitor:

• Anything inside a Literal is ignored.
  • This fixes all of the above examples.
• Failures to parse a string inside an annotation as Python code are simply ignored.
  • This will take care of any other usages of non-code strings inside an annotation.

Fixes:

• RemoveImportsVisitor crashes when parsing Literal["r+"] #924
• Literal values "and" and "or" cause ParserSyntaxError pydantic/bump-pydantic#124
• Literals trigger hard to understand errors pydantic/bump-pydantic#115

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[zsol]

Thank you!