fix: should count in module scope variables

[fi3ework]

What kind of change does this PR introduce?

Fix some issues introduced in https://github.com/webpack/webpack/pull/18348.

• include existing module identifiers to used names to avoid conflict after renaming symbols.
• refactor some logic to make it cleaner.
• correct the typo in the test case path.

Did you add tests for your changes?

Yes, in test/configCases/output-module/inlined-module/index.js, adding a index_value identifier which value will be renamed to, this will break the runtime before the PR as we didn't count existing identifiers in.

Does this PR introduce a breaking change?

No.

What needs to be documented once your changes are merged?

No.

[webpack-bot]

For maintainers only:

• [ ] This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• [ ] This needs to be backported to webpack 4 (issue will be created when merged)