Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

When a fish login shell is invoked within an existing login shell, avoid
duplicating path to the shims by removing existing occurrences before
prepending.

* Fix tests failing after #423

https://github.com/pyenv/pyenv-installer/pull/109

Otherwise, there's no effect for non-login interactive shells

also cleanup compatibility paths

pyenv-virtualenvs could not list conda environments & pyenv shell would only activate the base conda environment
the conda detection criteria of testing the presence of `conda` or `activate` files under `$(pyenv root)/versions/$version/bin` appears to be the culprit, since newer environments no longer include these files: those files reside in the base conda environment
- add detection criteria of `$(pyenv root)/versions/$version/conda-meta`
- compute the real prefix to the base environment from `realpath $(realpath $(pyenv root)/versions/$version)/../..`
- to allow that, enhance substitute `realpath` in `pyenv-virtualenvs` to reduce relative paths `.` & `..`, and factor that code out to a file under `libexec` for reuse
- hook `which` to locate conda from the real prefix

Manage `GET_PIP_URL` value at single place for readability.

Conda 4.4.4 (https://github.com/conda/conda/releases/tag/4.4.0)
made some changes on how to set up conda in the shell.
This commit addresses these.

This fixes some of the discussion in #178.

fixes #161, fixes #187

Use `set -gx` instead of `setenv` for fish shell.

Fix test "activate if the first virtualenv is a virtualenv": it was asserting
the output was of bash form, but not specifying that in the test — it now does.

Fixes https://github.com/pyenv/pyenv-virtualenv/issues/216.

* https://github.com/yyuu/pyenv/pull/615
* https://github.com/fish-shell/fish-shell/issues/310