fix: when parsing map length entries look for the key exactly equalling %

[sbogacz]

fix: when parsing map length entries look for the key exactly equalling % rather than being prefixed with it, to allow for map keys names to start with %.

When working with a custom provider I ran into an issue where map entries starting with % were being dropped, and not being passed to the provider implementation. I noticed that there were different approaches to checking for the map length, e.g. in internalconfigs/hcl2shim/flatmap.go:

https://github.com/hashicorp/terraform-plugin-sdk/blob/651ff3354401b964d3d3a9cd3d396afad121128c/internal/configs/hcl2shim/flatmap.go#L284

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