Use xdg instead of directories-next; correct typo in directory name

[smoelius]

This PR has two commits:

• The first replaces directories-next with xdg. (The former seems to have an uncertain future.)
• The second corrects a typo I made in Add --no-dev option #93.

EDIT: I could make these two PRs, or scrap either change, if you prefer.

[Shnatsel]

The dependency change and typo fix look good. Why does this also rename a directory and update references to it?

[smoelius]

Why does this also rename a directory and update references to it?

In #93, I created a directory named dep_tests. But that directory is referenced by tests in a module called deps, not dep. Hence, the directory should rightfully be called deps_tests, not dep_tests.

Sorry to be OCD, but it has irked me. And since I was submitting a PR anyway, I thought I would fix it.

[Shnatsel]

I see. What is the typo fix? I cannot spot it in the diff.

[Shnatsel]

Also, CI fails. Please run rustfmt to format the code. If that doesn't help please let me know - it's possible that CI version of rustfmt is too old.

[smoelius]

I see. What is the typo fix? I cannot spot it in the diff.

Naming the directory dep_tests instead of deps_tests is what I am calling a "typo." The fix is to rename the directory to the latter. (Sorry for being unclear.)

[Shnatsel]

Ah I see. Thanks! Merging.