Problems:
```
1) Psalm\Tests\MixinsDeepTest::testValidCode with data set "NamedMixinsWithoutT_WithObjectMethods"
Psalm\Exception\CodeException: MixedAssignment - src/somefile.php:22:1 - Unable to determine the type that $a is being assigned to

2) Psalm\Tests\MixinsDeepTest::testValidCode with data set "NamedMixinsWithT_WithObjectMethods"
Psalm\Exception\CodeException: MixedAssignment - src/somefile.php:37:1 - Unable to determine the type that $a is being assigned to
```

Creating the `handleMixins` method is necessary so that it can be used
in the future for recursive calls from the `handleTemplatedMixins` and
`handleRegularMixins` methods.
A recursive call to this method will allow us to search the depth of
the mixins.

Deep processing of mixins is planned to be done through recursive calls
to the `handleMixins` method.

To realise this, we need this method to return information that it has
managed to find the desired method.

Therefore, the `method_exists` variable was declared to make it clear
that the method was found.

The existing `naive_method_exists` variable is not suitable for us,
because it is not set to `true` if the method is magic (pseudo).

Extracting variables defined in the "if" statement block to the upper lines.

All this is necessary to use these variables for further searching
at the next deeper level of mixins, passing them to the `handleMixins`
method, performing recursion in this way.

Extracting code from:
```
if ($codebase->methods->methodExists(...)) {
    // here extracted
}
```

Problems:
```
1) Psalm\Tests\MixinsDeepTest::testValidCode with data set "TemplatedMixins_WithObjectMethods"
Psalm\Exception\CodeException: MixedAssignment - src/somefile.php:25:1 - Unable to determine the type that $a is being assigned to
```

Extracting code from:
```
if ($codebase->methods->methodExists(...)) {
    // here extracted
} elseif (...) {
    // here extracted
}
```

Comparing the `handleTemplatedMixins` and `handleRegularMixins` methods,
we can notice that in the `handleRegularMixins` method the `fq_class_name`
variable is overridden if the sought method is found in the mixin.

Whereas in the `handleTemplatedMixins` method this does not happen.
And I think therein lies the error.

Since the `class_storage` variable can be overridden in the
`handleTemplatedMixins` method, the `fq_class_name` variable should
also be overridden.

Problems:
```
1) Psalm\Tests\MixinsDeepTest::testValidCode with data set "NamedMixinsWithoutT_WithStaticMethods"
Psalm\Exception\CodeException: MixedAssignment - src/somefile.php:25:1 - Unable to determine the type that $a is being assigned to

2) Psalm\Tests\MixinsDeepTest::testValidCode with data set "NamedMixinsWithT_WithStaticMethods"
Psalm\Exception\CodeException: MixedAssignment - src/somefile.php:39:1 - Unable to determine the type that $a is being assigned to
```

Creating this method is necessary to be able to
make recursive calls using this method in the
next step.

Added recursive call

Problems:
```
1) Psalm\Tests\MixinsDeepTest::testValidCode with data set "NamedMixinsWithoutT_WithObjectProperties"
Psalm\Exception\CodeException: MixedAssignment - src/somefile.php:23:1 - Unable to determine the type that $a is being assigned to
```

Added recurcive call