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.

Some tricky changes in there but should be okay

This adds tracking of package hash in the in-memory index rather than
the current `undefined' values.

According to the test added, this is not necessary for transitive
package dep hash chcking, but does result in a more complete index search
result when doing app lookups, and could yield some optimizations on
hash checks by checking from the index structure before fetching a package.

Aliasing only had a bit of ad-hoc support in rebar3, and various issues
have encountered problems related to the package names not mapping
properly with the application name. One such issue is
https://github.com/erlang/rebar3/issues/1290

The problem has been hard to find because it only impacts transitive
dependencies (not top-level ones) of other packages. The root cause for
this is that the application name was not being tracked by rebar3's internal
index, only the package name and its version were.

When a given application was a package app, the data for the application
name would be reconstructed from the lock file, but only if it were a
top-level app or a dependency of a source application where parsing the
lock file is necessary to know what comes next.

When a transitive dependency of a package dependency was fetched, we
instead read its dependencies directly from the in-memory package index
within rebar3. This caused us to only read the package name and version,
and lost all information regarding application name. This worked fine
for most cases since for the vast majority of packages, the package name
matches the app name, but failed for all aliases, which would then be
moved to directories that wouldn't match the app name.

This in turn broke some aspects of code analysis (in Dialyzer), or other
functionality relying on static paths, such as including .hrl files from
dependencies.

This patch reformats the internal storage format of dependencies to
align with the internal one used by rebar3, so that the app name can be
carried along with the package name and its version.

The fix can only work once `rebar3 update` is called so the index is
rebuilt internally, and will the file cached on disk will be
incompatible with older rebar3 versions.

Currently, the following is not covered:

 - Tests
 - Including the package hashes of dependencies so they may match what
   is in a lock file -- they're being `undefined` instead, which may
   break some lookups. The previous format did not lend itself to hashing
   in the same way, and it is possible transitive deps were not being
   tracked properly, or worked by respecting the current package hierarchy.
   This will require further analysis

For now this commit can allow reviewing and discussion.

Add more version constraints

Allow for any number of whitespaces after compairison opperator

Improve updating and error printing

Fix failing tests

- Crashes in providers lib when no providers in a namespace are bare
- Making sure bareness matches semantics; i.e. a bare provider is
  visible, a non-bare provider is hidden.

- The rebar package index files have been moved off the default path and
  will require a new `rebar3 update`
- Caching of downloaded packages automatically takes place in a path
  relative to the CDN used
- The cache path is not shared with hex as we now write and modify data
  in there arbitrarily
- Basic tests plus the working set for more of them is included

Also expose it in rebar_api