Under PEP-517, installing build dependencies is up to the frontend.

Closes gh-2303

As setuptools is now python 3.5+, this compatibility shim is no longer
necessary.

Some setup.py scripts, use sys.argv[0] to locate the source directory of
a project. I added this to build_meta.__legacy__ since that is focused
on backwards compatibility with old scripts. However, @pganssle said
this behaviour should not be added to setuptools.build_meta. Fixes #1628

Fix the following cases:

* `build_sdist` is called with another sdist already present
  in the destination directory

* `build_wheel` is called with the destination directory not
  already created

`build_meta.build_wheel` assumes that the only wheel in its output
directory is the one it builds, but prior to this, it also used the
`dist/` folder as its working output directory. This commit uses a
temporary directory instead, preventing an error that was triggered when
previously-generated wheel files were still sitting in `dist/`.

Fixes GH #1671

Since pkg_resources is imported elsewhere anyway, we don't get much
value out of porting the requirement parser locally.

This fixes GH #1682 by porting the pkg_resources requirement parsing
logic into setuptools.build_meta, so that all valid requirement
specifiers passed to setup_requires will be added to the
get_requires_for_build_* function outputs.

Fixes GH #1682