Normalize relative paths to an absolute path

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Description

This issue regarding dereferencing symlinks has plagued Terraform v1.3.0+ (reported for several versions here). The first issue, dereferencing symlinks whose targets resolve to symlinks outside of the src directory was resolved in #37. The second issue, when the src directory itself is a symlink was resolved in #36. The last issue, and the focus of this PR, deals with normalizing paths when attempting to traverse the src directory. My change is simple: before attempting to traverse the src normalize the path representation to its absolute form.

Big thank you to @willhughes-au and @calvinbui for providing the fixture used to test that paths are normalized.

If you'd like to test that this works with Terraform, you will need to go install . from main, replacing the indirect dependency for go-slug with a repo containing these changes.

cd /path/to/terraform
go mod edit -replace github.com/hashicorp/go-slug=/path/to/your/go-slug
go install .