tools/importer-rest-api-specs - fix possible const conflict

[teowa]

resolve issue in PR #4374
fix the issue that strings.EquelFold will randomly choose from capitalized or lowercased models, thus leads to conflicts in Enums, the details are in #4374 (comment)
By my local test, we can use exact match here without breaking existing generated data.

[teowa]

remove findAndMergeLocalMixins in first time parsing specs, by test this can avoid generate two provisioningState consts after flatten. For existing data, this won't break any thing, and it should save some time.
https://github.com/hashicorp/pandora/blob/22811ab2048bd6af4af2814f0d462b9f60eff809/tools/importer-rest-api-specs/components/parser/flattener.go#L30-46

[teowa]

superseded by #4411