standard_module_structure: fix false positives when passing a directory

[bendrucker]

As reported in #834, the terraform_standard_module_structure was reporting false warnings when TFLint was invoked with a non-cwd directory:

$ /usr/local/bin/tflint -c /tmp/.tflint.hcl /tmp/ok
2 issue(s) found:

Warning: output "test" should be moved from /tmp/ok/outputs.tf to outputs.tf (terraform_standard_module_structure)

  on /tmp/ok/outputs.tf line 1:
   2: output "test" {

Reference: https://github.com/terraform-linters/tflint/blob/v0.17.0/docs/rules/terraform_standard_module_structure.md

Warning: variable "test" should be moved from /tmp/ok/variables.tf to variables.tf (terraform_standard_module_structure)

  on /tmp/ok/variables.tf line 1:
   1: variable "test" {

Reference: https://github.com/terraform-linters/tflint/blob/v0.17.0/docs/rules/terraform_standard_module_structure.md

Directories were properly handled when checking that the required files were present:

tflint/rules/terraformrules/terraform_standard_module_structure.go

Lines 62 to 65 in e87fc47

	files := runner.Files()
	for name, file := range files {
	files[filepath.Base(name)] = file
	}

The logic that checked parsed variables/outputs for their filenames was missing that same logic.

Before including the fix, the modified test validates the bug, failing because of an unexpected issue:

--- FAIL: Test_TerraformStandardModuleStructureRule (0.00s)
    --- FAIL: Test_TerraformStandardModuleStructureRule/directory_in_path (0.00s)
        testing.go:83: Expected issues are not matched:
               tflint.Issues{
              	&{Rule: &terraformrules.TerraformStandardModuleStructureRule{}, Message: "Module should include an empty outputs.tf file", Range: {Filename: "foo/outputs.tf", Start: {Line: 1, Column: 1}}},
            + 	&{
            + 		Rule:    &terraformrules.TerraformStandardModuleStructureRule{},
            + 		Message: `variable "v" should be moved from foo/variables.tf to variables.tf`,
            + 		Range:   s"foo/variables.tf:2,1-13",
            + 	},
              }
            
FAIL
FAIL	github.com/terraform-linters/tflint/rules/terraformrules	0.391s
FAIL

Closes #834