rebar_deps: simplify preprocess/2

Move skip_deps= check into a separate funtion to simplify preprocess/2.
rebar_deps: fix minor issues introduced in a26acb2f

- restore meaningful blank lines
- is_app_avilable/2: use spaces in record syntax to match surrounding
  code
- use simpler is_raw #dep{} record field match
- fix minor typo
rebar_deps: simplify is_app_available/2

Replace deeply nested case of expressions with separate functions.
Fix single overlong line introduced in 0b833391