python_pep517: use installer instead of pip

Instead of battling pip to install a wheel, use installer. Installer
does one thing, so it's faster and easier to work with.

This means setuptools, pip, and wheel are no longer part of the
bootstrap phase, so they can be built normally.  To avoid sysroot file
conflicts these three recipes can't install .pyc files to the native
sysroot.

We currently patch pypa/installer to allow us to override the interpreter
used, which means we can drop the interpreter seding.

We don't need to recompile any Python which is found in $bindir as
Python doesn't actually load those files.

Across a build of oe-core, the only differences between using pip and
installer are:

- the .dist-info/RECORD files are ordered differently
- the .dist-info/REQUESTED and INSTALLER files are not created
- the hashbang in native scripts is "/usr/bin/env nativepython" instead
  of pointing directly at the native sysroot python3.

(From OE-Core rev: f780f6d920d8bbfb674d6066a8b899417decf8d2)

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-devtools/python/python3-flit-core_3.7.1.bb

@@ -13,7 +13,7 @@ SRC_URI[sha256sum] = "3c9bd9c140515bfe62dd938c6610d10d6efb9e35cc647fc614fe5fb3a5
 inherit pypi flit_core
 
 # Need to install by hand as there's a dependency loop
-DEPENDS:remove:class-native = " python3-pip-native"
+DEPENDS:remove:class-native = " python3-installer-native"
 DEPENDS:append:class-native = " unzip-native"
 
 # We need the full flit tarball