* Report error when not sourcing activate scripts (#1372)

The activate scripts are to be used in the context of the current shell session.

The feature so far only works in powershell and bash, because in other shells it
is hard to get the name of the currently executing script.

Also added tests.

* fix

Fix PSScriptAnalyzer warnings, and follow style from
-Setting CodeFormatting

Closes https://github.com/pypa/virtualenv/issues/1371

See history of [bskinn/virtualenv:all-prompts branch](https://github.com/bskinn/virtualenv/commits/all-prompts), tip at
bskinn/virtualenv@67cda5f, for the actual sequence of
development of the prompt tests, leading to the first commit of the PR (e6a2b99)

* Always store _OLD_VIRTUAL_PATH in a sh compat format

When using a subshell from fish while within a virtualenv the
_OLD_VIRTUAL_PATH variable ends up in fish format and not the format of
the subshell. This breaks the first step of the activate script where it
attepts to deactivate the previous environment.

This patch will handle conversion to and from sh and fish formats within
the activate.fish script

* Add test and update to new delimiter

* Fix lint

Remove line that didn't lint as we don't need it.

* Rebuild virtualenv.py

* Add changelog

* fix_lint

* Try to add fish to build env

* Use less fish (only when running py tests)

* remove code duplication for activation testers

* Consolidate activation testing

- add tests for csh, bash
- rework bash, csh tests
- remove powershell singing
- powershell pydoc support

* file and with extension

* universal newline

* Add debug env var, and normalize on Linux to via realpath (Jython failure)

* switch to bat

* fix xonosh

* fix xonsh further

* fix jython

* fix lint

* enable pytest debug

* Added activate.ps1 to list of generated activation scripts
* extended tests to cover cmdline
* Use path_locations to test directories
* Add tests for powershell activation
* Fix environment creation for cmdline tests
* pwsh.exe on Windows
* make executable dependent tests optional locally
* fixes
* fix path normalization and casing
* turn path to long
* debug
* fix documentation