Incorrect use of PIP_EXTRA_INDEX_URL

[hhoeflin]

I used the plugin and it provides incorrect setting for PIP_EXTRA_INDEX_URL. If it is found, it sets PDM_PYPI_URL to that value, but that is wrong, as this drops the default pypi-url from the search. as the variable says, it is an extra index url and so should not drop the default.

[hhoeflin]

It would also be nice if the log-messages the plugin writes are prefaced by the plugin-name. It took me a while to figure out where the problem comes from.