Scanner: first scanner, missing Repo arg

This is the first scanner implementation. 

Just a small question, from where should be extract the VCS urls?

Input:
```sh
src tool sourcegraph.com/sourcegraph/srclib-php scan
```

Output:
```json
[
    {
        "Name": "sourcegraph/srclib-php",
        "Type": "ComposerPackage",
        "Globs": [],
        "Files": [
            "src/Sourcegraph/PHP/Command/ScannerCommand.php",
            "src/Sourcegraph/PHP/Grapher/DefExtractor.php",
            "src/Sourcegraph/PHP/Grapher/DocExtractor.php",
            "src/Sourcegraph/PHP/Grapher/RefExtractor.php",
            "src/Sourcegraph/PHP/Grapher.php",
            "src/Sourcegraph/PHP/NodeVisitor/NodeCollector.php",
            "src/Sourcegraph/PHP/Scanner/SourceUnitBuilder.php",
            "src/Sourcegraph/PHP/Scanner.php",
            "tests/bootstrap.php",
            "tests/fixtures/graph/000.invalid.php",
            "tests/fixtures/graph/001.functions.php",
            "tests/fixtures/graph/002.classes.php",
            "tests/fixtures/graph/003.constants.php",
            "tests/fixtures/graph/004.properties.php",
            "tests/fixtures/graph/005.traits.php",
            "tests/fixtures/graph/006.interfaces.php",
            "tests/fixtures/graph/101.params.php",
            "tests/fixtures/graph/102.uses.php",
            "tests/fixtures/graph/103.inherance.php",
            "tests/fixtures/graph/104.asigns.php",
            "tests/fixtures/graph/500.complex.php",
            "tests/fixtures/scanner/monolog/src/Monolog/Logger.php",
            "tests/Sourcegraph/Tests/PHP/Grapher/DefExtractorTest.php",
            "tests/Sourcegraph/Tests/PHP/Grapher/DocExtractorTest.php",
            "tests/Sourcegraph/Tests/PHP/Grapher/RefExtractorTest.php",
            "tests/Sourcegraph/Tests/PHP/GrapherTest.php",
            "tests/Sourcegraph/Tests/PHP/Scanner/SourceUnitBuilderTest.php",
            "tests/Sourcegraph/Tests/PHP/ScannerTest.php",
            "tests/Sourcegraph/Tests/TestCase.php"
        ],
        "Dependencies": [
            "nikic/php-parser",
            "symfony/console",
            "phpunit/phpunit"
        ],
        "Data": {
            "namespaces": [
                "Sourcegraph\\PHP"
            ]
        },
        "Ops": {
            "depresolve": null,
            "graph": null
        }
    }
]
```