only insert eggs ahead of parent if not already on path
only insert eggs ahead of parent if not already on path

in `insert_on(replace=False)`

and set `replace=False` by default in `activate()`

Fixes pkg_resources modifying sys.path for all eggs
with SETUPTOOLS_SYS_PATH_TECHINQUE=raw
pypa/setuptools,pypa/setuptools,pypa/setuptools