Didn't handle tuple-based dep definitions right

Upgrading dependencies already checked for package updates before running
the upgrade task. Plugins didn't have that mechanism in place and that
made upgrading plugins over hex trickier since users could be stuck a
long time without running an update manually.

This fix mostly readapts existing dep code to the context of plugins to
look them up one at a time and run the update as part of the upgrade.

this fixes the issue where using 'rebar3 plugins upgrade P'
would result in a plugin's .app file having an empty
modules list. The code this replaces hadn't been touched
since rebar3 3.0.0-beta.3 and a lot of improvements have
gone into dep handling and the compiler since then. This
change should take advantage of those changes.

This is done through 3 main change groups:

- replacing `~s` by `~ts` in format strings, so that strings that
  contain unicode are properly printed rather than crashing
- adding the `unicode` argument to all function of the `re` module to
  ensure transformations on strings containing unicode data are valid
  instead of crashing (see issue #1302)
- replacing `ec_cnv:to_binary/1` and `ec_cnv:to_list/1` with matching
  functions in `rebar_utils`.

The last point has been done, rather than modifying and updating erlware
commons, because binary and list conversions can be a contentious
subject. For example, if what is being handled is actually bytes from a
given binary stream, then forcing a byte-oriented interpretation of the
data can corrupt it. As such, it does not appear safe to modify erlware
commons' conversion functions since it may not be safe for all its
users.

Instead, rebar3 reimplements a subset of them (only converting
atoms and chardata, ignoring numbers) with the explicit purpose of
handling unicode string data.

Tests were left as unchanged as possible. This may impact the ability to
run rebar3's own suites in a unicode path, but respects a principle of
least change for such a large patch.