Fixes #2042

This PR moves the examples from the {project_dir}/examples to {project_dir}/pyqtgraph/examples.

This change was prompted by #2042 which @ksunden pointed out that some of the examples didn't work when pyqtgraph was installed in non-editable mode and the examples were run from outside the project directory. Given the extensive path manipulation via import initExample on every example, and in other places, figured this would be a good time to take out some of the custom code written for pyqtgraph's setup.py and use standard setuptools functionality instead.

I also took the opportunity to update MANIFEST.in which only effects source distributions.

A by-product of this change means that examples may not work if pyqtgraph is not installed via pip. Given that I'd like to phase out support for pyqtgraph to be used when not via a package manager, I'm good with this change (but others may not be).

The testing I did was as follows

I launched the example app via python -m pyqtgraph.examples and then ran the parameter tree example ensuring it ran successfully (this example is good because it imports other python modules in the example directory, and imports from pyqtgraph)

I did this for editable and non-editable installs, from both the root project directory and a directory not within the project.

Furthermore, I ran through the test suite pytest -n auto pyqtgraph/examples

What no longer works is running python -m pyqtgraph.examples from the project directory (example app pops up) but almost all examples fail as they attempt to import pyqtgraph which reports package not found. this works now

The only use-case I can identify where examples do not work is if a user attempts to run an example directly (ex: python pyqtgraph/examples/PlotSpeedTest.py and pyqtgraph is not installed; the user will get an ImportError.

EDIT: slight oddity; somewhere in the development process here, I managed to break pyopengl to the point that I would get an import error; after force-reinstalling it, I could no longer replicate

Python 3.9.7 (default, Sep 17 2021, 12:53:27)
[Clang 12.0.5 (clang-1205.0.22.11)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> from OpenGL.GL import *
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/ogi/.zinit/plugins/pyenv---pyenv/versions/3.9.7/envs/pyqtgraph-pyqt_62-py39/lib/python3.9/site-packages/OpenGL/GL/__init__.py", line 3, in <module>
    from OpenGL import error as _error
  File "/Users/ogi/.zinit/plugins/pyenv---pyenv/versions/3.9.7/envs/pyqtgraph-pyqt_62-py39/lib/python3.9/site-packages/OpenGL/error.py", line 12, in <module>
    from OpenGL import platform, _configflags
  File "/Users/ogi/.zinit/plugins/pyenv---pyenv/versions/3.9.7/envs/pyqtgraph-pyqt_62-py39/lib/python3.9/site-packages/OpenGL/platform/__init__.py", line 21, in <module>
    from OpenGL import _configflags
  File "/Users/ogi/.zinit/plugins/pyenv---pyenv/versions/3.9.7/envs/pyqtgraph-pyqt_62-py39/lib/python3.9/site-packages/OpenGL/_configflags.py", line 2, in <module>
    from OpenGL import (
ImportError: cannot import name 'ERROR_CHECKING' from 'OpenGL' (unknown location)
>>> exit()