copy languages package from Sourcegraph to Zoekt

[stefanhengl]

We want Zoekt and Sourcegraph to use the same language package. In this PR we move the languages package from Sourcegraph to Zoekt, so that Zoekt can use it and Sourcegraph can import it.

Notes:

• Zoekt doesn't need to fetch content async which is why I added a little helper func GetLanguagesFromContent to make the call sites in Zoekt less awkward.
• Sourcegraph's languages package always classified .cls files as Apex, while Zoekt did a content based check. With this PR we follow Zoekt's approach. Specifically, I removed .cls from unsupportedByEnryExtensionToNameMap. I added an additional unit test to cover this case.

Test plan:
I appended the test cases from the old Zoekt languages packages to the tests I copied over from Sourcegraph

[varungandhi-src]

Left one minor comment.

[varungandhi-src]

I think we can simplify the logic a bit be something like:

var content []byte
if doc.SkipReason == SkipReasonNone {
    content = doc.Content
}
langs := languages.GetLanguagesFromContent(doc.Name, content)
if len(langs) > 0 {
    doc.Language = langs[0]
}

[keegancsmith]

One minor request: can you add a package docstring. Given this is now an exported api seems useful to ensure everything looks good via go doc -all ./languages. Additionally I notice that NormalizeLanguage has no docstring.