Make clear in PEP 503 that search is not supported.

[stephen-dexda]

Environment

• pip version: 19.1.1
• Python version: 3.7

Description
With a custom PEP 503 repo, pip search -i <repo-root> foo fails with:

ERROR: HTTP error 404 while getting <repo-root>RPC2
ERROR: Exception:
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/cli/base_command.py", line 178, in main
    status = self.run(options, args)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/commands/search.py", line 48, in run
    pypi_hits = self.search(query, options)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/commands/search.py", line 65, in search
    hits = pypi.search({'name': query, 'summary': query}, 'or')
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/xmlrpc/client.py", line 1112, in __call__
    return self.__send(self.__name, args)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/xmlrpc/client.py", line 1452, in __request
    verbose=self.__verbose
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_internal/download.py", line 832, in request
    response.raise_for_status()
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/site-packages/pip/_vendor/requests/models.py", line 940, in raise_for_status
    raise HTTPError(http_error_msg, response=self)
pip._vendor.requests.exceptions.HTTPError: 404 Client Error: Not Found for url: <repo-root>RPC2

(Similar failure if rooted at /simple/, except it tries to POST to /simple/ instead of /RPC2).

Per #5216 it seems that search doesn't just walk the root and/or subdirs (which might be assumed), and uses a non-PEP 503 mechanism instead.

Expected behavior
This is documented, so the obvious pip search is not used by someone trying to test their implementation ;)

[pradyunsg]

Happy to accept a PR adding this information to pip search's reference.

[stephen-dexda]

Ideally this would show up in the "pip search" error message if the user tried to search a custom repo and POSTing failed (along the lines of "if is a PEP 503 simple repo, search is not supported for that repo type"). The user may be unlikely to look in the search docs when presented with that error

[pradyunsg]

Closing this since there's been no movement on this in quite a while, and since pip search is no longer usable on PyPI: #5216