…entiate from attr (ostrolucky)

This PR was merged into the 4.3-dev branch.

Discussion
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[Translation] Add XLIFF 1 source to metadata to differentiate from attr

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

Here's our use case this patch solves:

We have 2 translation files. One is created by developer and has empty `<target>`. Second one is created by translator and has non-empty `<target>`, but same `<source>`.

Now, unlike Symfony fixtures, we also use `resname=` XLIFF1 attribute. This allows us to detect when translation is out of date, which happens when `<source>` in dev translation is changed, but `resname=` is not. As visualization which better illustrates problem:

dev.xlf:
```xml
<trans-unit id="source_is_different" resname="source_is_different">
    <source>Welcome to Calida</source>
</trans-unit>
```
real.xlf:
```xml
<trans-unit id="source_is_different" resname="source_is_different">
    <source>Welcome to Jacqueline</source>
    <target>Willkommen bei Jacqueline</target>
</trans-unit>
```

We need to be able to tell `<source>` for this pair is different (so we prepend alert glyph to translation message, to alert developer something is wrong). In current XliffFileLoader there is no way for consumer to figure out if translation key came from `resname` or `<source>`, so we had to copy whole loader.

Commits
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ab04f25 [Translation] Add XLIFF 1 source to metadata to differentiate from attr