build-dep: parse deps directly from template

There is a chicken-and-egg problem related to #24 (closed): Knot projects' archive creation requires certain subset of build deps and apkg build-dep works on source package which can't be created without archive.

In order to break the loop, apkg build-dep should be extended with support for parsing build deps directly from package templates.

In fact it should operate on rendered templates by default because it's faster and requires less deps due to skipping srcpkg build.

-s/--srcpkg option to install from srcpkg (current behavior) should be provided so that it's possible to install build deps both from templates and source packages.

Finally, --list option to only list deps without installation might come in handy, consider adding that too.