Remove staging directory paths required by old pip install method.
Add comment to explain why the bin path although unused has been
retained.

Relates pypa/pip#5983, #475, #503

Update the pip install arguments to use a specific target directory
to install the requested library. Set the install command to
use PEP 517 for building source distributions to ensure a wheel
is always built before installation.

Relates pypa/pip#4501, #475, #503

Build a wheel before installation if the requested package
does not already have a .whl file. No longer check for egg
distributions.

Relates #475, #503

Stop checking for dependencies using run requires.

Relates #475, #503

Use metadata attributes to retrieve core data such as the
name, version and summary of the requested package.

Relates https://www.python.org/dev/peps/pep-0426/#core-metadata, #475, #503

Use wheel dist-info metadata and record files to perform the
installation of the requested package including all its
dependencies.

BREAKING CHANGE:

Currently rez is using `distlib` 0.2.4 released in late 2016. That
version of the library does not correctly detect installed
wheel distributions meaning that there is no access to metadata
and record files.

To fix your project, update your `distlib` library located in the
src/vendor directory.

Revert dependency gathering method because new API methods
such as distutils DependencyFinder and metadata extra, dependency
fields are still not well documented or widely adopted with
package maintainers.
Ensure each dependency is installed as an individual rez package
to avoid duplicates.

Relates nerdvegas#475, nerdvegas#503

The installation via Wheel distribution when using the --target
flag, expects the bin directory to be two directories from the base
path. After the wheel unpacking the bin directory is located in the
base directory. Move the bin directory where the RECORD file expects
it to be in order to improve readability and maintain consistency.

rez will now enforce wheel distribution based installations. A version
of pip >= 19 is required in order to do so. Add pip version check
to the callbacks of rez-pip command to validate that an up to date
pip is being used.

Relates: #602 #628 https://pip.pypa.io/en/stable/news/#id49