Port before v0.60/virtual_environments (#849)

This PR is part of porting all old scripts #221 and includes the
`virtual_environments` module

## 5 files changed

- `conda.nu`: removed, already in
`modules/virtual_enviromnents/conda/nu_conda.nu`
- `conda_deactivate.nu`: also removed, has already been merged with
`nu_conda.nu`
- `README.nu`: removed, similar info in
`modules/virtual_enviromnents/README.md`

- `venv.nu`: ported to `modules/virtual_enviromnents/venv/venv.nu`
- `venv_deactivate.nu`: ported to
`modules/virtual_enviromnents/venv/venv_deactivate.nu`

modules/virtual_environments/venv.nu

@@ -0,0 +1,25 @@
+export def --env activate [venv_dir] {
+    let venv_abs_dir = ($venv_dir | path expand)
+    let venv_name = ($venv_abs_dir | path basename)
+    let old_path = $env.PATH
+    let new_path = (venv-path $venv_abs_dir)
+    let new_env =  ({VENV_OLD_PATH: $old_path, VIRTUAL_ENV: $venv_name} | merge $new_path)
+    load-env $new_env
+}
+
+def "venv-path" [venv_dir] {
+      let env_path = [
+        $venv_dir,
+        ([$venv_dir, "bin"] | path join)
+      ]
+      return {
+        PATH: ($env.PATH | prepend $env_path)
+      }
+}
+
+export def --env deactivate [] {
+    $env.PATH = $env.VENV_OLD_PATH
+    hide-env VIRTUAL_ENV
+    hide-env VENV_OLD_PATH
+
+}