runner: clean Terraform source path for comparison

[bendrucker]

When TFLint is called with a relative path formatted with a leading ./, results are empty from Files(). The logs I added for the rule that reproduced this printed:

$ tflint ./my/module
...
19:14:55 terraform_standard_module_structure.go:73: [DEBUG] 0 files found: map[]

Clean is called internally by Dir, meaning the comparison was "my/module" == "./my/module". Cleaning both paths ensures the values are normalized for comparison.

The tests methods also call Dir to process a map of path -> file so adding a basic test didn't actually add coverage.

But I did verify that it works as expected:

19:17:00 terraform_standard_module_structure.go:73: [DEBUG] 6 files found: map[backend.tf:0xc0001a0b40 main.tf:0xc0001a0f00 outputs.tf:0xc0001a0f80 providers.tf:0xc0001a1000 variables.tf:0xc0001a1080 versions.tf:0xc0001a0ac0]

[bendrucker]

This will affect any rule that uses Files() for users passing in a path (typically monorepos)