Add alternative configurations, add throwing exceptions to Config class

[devanych]

Q	A
Is bugfix?	❌
New feature?	✔️
Breaks BC?	❌
Fixed issues	#14

Alternative configurations complement (do not override) the root configuration.

Adding it to composer.json for the root package:

"config-plugin-environments": {
    "alfa": {
        "params": "config/alfa/params.php",
        "web": "config/alfa/web.php",
        "main": [
            "$web",
            "config/alfa/main.php"
        ]
    }
},

Getting the configuration of the alternative build group:

$config new Config($rootPath, $configsPath, 'alfa');

$config->get('main');

Getting the configuration of the root build group:

$config new Config($rootPath, $configsPath);
$config->get($groupName);

[samdark]

@yiiliveext suggests to rename "alternatives" to "environments".

[vjik]

Don't work if application use option source-directory.

Error:

λ yii serve

Fatal error: Uncaught ErrorException: The "/" configuration build does not exist. in E:\open-source\yiisoft\yii-dev-tool\dev\config\src\Config.php on line 365

ErrorException: The "/" configuration build does not exist. in E:\open-source\yiisoft\yii-dev-tool\dev\config\src\Config.php on line 365

Call Stack:
    0.0003     405976   1. {main}() E:\open-source\yiisoft\yii-dev-tool\dev\demo\yii:0
    0.0538    4844960   2. Yiisoft\Config\Config->get($group = 'console', $build = ???) E:\open-source\yiisoft\yii-dev-tool\dev\demo\yii:22
    0.0538    4844960   3. Yiisoft\Config\Config->buildGroup($group = 'params', $build = '/', $rootBuildGroupConfig = ???) E:\open-source\yiisoft\yii-dev-tool\dev\config\src\Config.php:68
    0.0538    4844960   4. Yiisoft\Config\Config->checkBuildGroup($group = 'params', $build = '/') E:\open-source\yiisoft\yii-dev-tool\dev\config\src\Config.php:101
    0.0538    4845280   5. Yiisoft\Config\Config->throwException($message = 'The "/" configuration build does not exist.') E:\open-source\yiisoft\yii-dev-tool\dev\config\src\Config.php:314

[devanych]

Don't work if application use option source-directory.

Did you regenerate merge_plan.php?

This PR changes its format.

[vjik]

Don't work if application use option source-directory.

Did you regenerate merge_plan.php?

This PR changes its format.

Oh, sorry. Yes, it's work.

[vjik]

When getting alternative configuration params from alternatives should merged with params from general configuration and then pass both to general configs and alternative configs.

Now merged params pass only to alternative configs. In general configs pass only general params. This is wrong behavior.

[devanych]

When getting alternative configuration params from alternatives should merged with params from general configuration and then pass both to general configs and alternative configs.

Now merged params pass only to alternative configs. In general configs pass only general params. This is wrong behavior.

Done.

[devanych]

@yiiliveext suggests to rename "alternatives" to "environments".

Done.

[samdark]

👍