relative-url-prefix errors on any relative links that are not at the same depth

**Describe the bug**

When the rule `relative-url-prefix` is enabled it considers deep relative URLs to **not** be prefixed correctly.

```TypeScript
// Codelyzer considers valid:
'./foo.ts'

// Codelyzer considers invalid:
'./../foo.ts'
```

While this seems like a bug to me since `./../foo.ts` **is** a relative URL, this library is explicitly testing for this behavior so it may not be considered a bug by the maintainer(s). Reference: https://github.com/mgechev/codelyzer/blob/f12f27b699187bbb443a0400ab678b8ec911eb26/test/relativeUrlPrefixRule.spec.ts#L48-L61

**Context and configuration**

Please share:
- Which rule is causing the problem
- `relative-url-prefix`
- What's the content of your `tsconfig.json`
- _see below_
- What's the content of `~/.codelyzer.js` (if applicable)
- n/a

TSConfig.json Contents

```
{
"angularCompilerOptions": {
"fullTemplateTypeCheck": true,
"preserveWhitespaces": false
},
"compileOnSave": false,
"compilerOptions": {
"allowJs": true,
"alwaysStrict": true,
"baseUrl": "demo",
"declaration": false,
"emitDecoratorMetadata": true,
"experimentalDecorators": true,
"lib": [
"es2016",
"es2015",
"dom"
],
"moduleResolution": "node",
"noImplicitAny": false,
"outDir": "./dist/out-tsc",
"plugins": [
{
"name": "typescript-tslint-plugin"
}
],
"skipLibCheck": true,
"sourceMap": true,
"strict": true,
"target": "es5",
"typeRoots": [
"node_modules/@types"
]
}
}
```

**To Reproduce**

Steps to reproduce the behavior.

**Expected behavior**

I would expect the following to raise the lint error:

```
'foo.ts'
'../foo.ts'
'@my/lib/foo.ts'
```

I would **not** expect the following to raise a lint error:

```
'./foo.ts'
'./../foo.ts'
```

**Code**

Share the code which will let us reproduce the problem

Examples above will reproduce the issue.

**Environment**

- Version [e.g. 22] `5.0.0`
- OS: [e.g. iOS] `10.14.4`
- Node.js version `11.9.0`
- Package manager (yarn/npm) version `yarn 1.15.2`
- Angular version `7.2.2`
- tslint version `5.14.0`

**Additional context**

Add any other context about the problem here.

Start with test/relativeUrlPrefixRule.spec.ts, especially the cases around lines 48-61, and run that rule's tests to confirm the current behavior. Update the rule and its tests so ./../foo.ts is accepted while foo.ts and ../foo.ts still raise the lint error.

Written by the indexing model from the issue text.