port conda to engine-q (#160)

engine-q/virtual_environments/README.md

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+# Virtual environment scripts
+
+The scripts in this directory activate virtual environments for Conda environments.  They follow the pattern described in the [Nushell 0.32
+changelog](https://www.nushell.sh/blog/2021-06-01-nushell_0_32.html#environment-loading-lily-mara):
+
+```
+$ load-env (activate some-env)
+```
+
+A custom command (`activate`) creates a table with environment variables and
+`load-env` is used to load it into the shell's scope.
+
+In most cases, deactivation is a matter of restoring the PATH variable to the
+state before activating the env and removing any additional variables. There are
+no environment-specific elements to this, which is why the same deactivation
+script can be used for deactivating any environment of a specific type.  For
+example, `source conda_deactivate.nu` will deactivate any Conda env, there are
+no input parameters.
+
+## Expected Usage
+
+1. Source an activation script in your `config.nu`. For example, `conda.nu`.
+   You'll then have the `conda-env` command available.
+2. Create an alias for sourcing the appropriate deactivation script:
+   `alias conda-deactivate = source /path/to/conda_deactivate.nu`
+3. Activate with `load-env (conda-env env-name)`. You might want to define some
+   shorter aliases for both commands, if typing that every time seems like a
+   hassle.
+4. If you're using [Starship](https://starship.rs/), your prompt should reflect the activated env.
+5. When done, deactivate with your alias: `conda-deactivate`.
+
+Look at the script files to find the exact command name for creating an environment table.
+
+## `conda.nu`
+
+Limitations:
+
+- The "root_prefix" might not actually correspond to the correct path to the Conda envs. You can fix
+  this for your setup by changing how the root prefix is found in the `conda-env` command.
+- Nested envs are not well supported. If you activate a Conda env while another one is
+  activated, new elements will be appended to the PATH, but the other environment
+  variables will be overwritten. There's no way to then restore the PATH to the state
+  it was in before activating the *first* env (at least not with this script directly).
+- The prompt is not updated by the script. Consider using [Starship](https://starship.rs/)
+  with the Python prompt element.

engine-q/virtual_environments/conda.nu

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+def conda-env [env-name] {
+    let conda-info = (conda info --envs --json | from json)
+    let suffix = (if $env-name == "base" {""} else {(["envs" $env-name] | path join)})
+    let env-dir = ([$conda-info.root_prefix $suffix] | path join)
+    let old-path = ($env.PATH | str collect (path-sep))
+    let new-path = (if (windows?) { (conda-create-path-windows $env-dir) } else { (conda-create-path-unix $env-dir) })
+    let new-env = {
+        CONDA_DEFAULT_ENV: $env-name
+        CONDA_PREFIX: $env-dir
+        CONDA_PROMPT_MODIFIER: $"[($env-name)]"
+        CONDA_SHLVL: "1"
+        CONDA_OLD_PATH: $old-path
+    }
+    $new-env | merge { $new-path }
+}
+
+def conda-create-path-windows [env-dir] {
+    # 1. Conda on Windows needs a few additional Path elements
+    # 2. The path env var on Windows is called Path (not PATH)
+    let env-path = [
+        $env-dir
+        ([$env-dir "Scripts"] | path join)
+        ([$env-dir "Library" "mingw-w64"] | path join)
+        ([$env-dir "Library" "bin"] | path join)
+        ([$env-dir "Library" "usr" "bin"] | path join)
+    ]
+    let new-path = ([$env-path $env.PATH] | flatten | str collect (path-sep))
+    {
+        PATH: $new-path
+    }
+}
+
+def conda-create-path-unix [env-dir] {
+    let env-path = [
+        ([$env-dir "bin"] | path join)
+    ]
+    let new-path = ([$env-path $env.PATH] | flatten | str collect (path-sep))
+    {
+        PATH: $new-path
+    }
+}
+
+def windows? [] {
+    (sys).host.name == "Windows"
+}
+
+def path-sep [] {
+    if (windows?) { ";" } else { ":" }
+}

engine-q/virtual_environments/conda_deactivate.nu

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+let path-name = (if ((sys).host.name == "Windows") { "Path" } else { "PATH" })
+let-env $path-name = $env.CONDA_OLD_PATH
+hide CONDA_PROMPT_MODIFIER
+hide CONDA_PREFIX
+hide CONDA_SHLVL
+hide CONDA_DEFAULT_ENV
+hide CONDA_OLD_PATH