builddep: added usage & summary, and fixed some PEP8 issues

Output:

```
$ dnf help builddep
builddep [PACKAGE.src.rpm|PACKAGE.spec]

Install build dependencies for .src.rpm or .spec file
```
```
$ dnf help
..
..
List of Plugin Commands

builddep               Install build dependencies for .src.rpm or .spec file
copr                   Interact with Copr repositories.
```