Begin implementing null backend

README.md

@@ -145,6 +145,9 @@ by setting the environment variable `JULIA_CONDAPKG_BACKEND` to one of the follo
   [MicroMamba.jl](https://github.com/cjdoris/MicroMamba.jl).
 - `System`: Use a pre-installed Conda. If `JULIA_CONDAPKG_EXE` is set, that is used.
   Otherwise we look for `conda`, `mamba` or `micromamba` in your `PATH`.
+- `Null`: Don't use `CondaPkg.jl` to manage dependencies,
+  eg. If you are using a pre-existing conda installation that satisfies the dependencies of your project.
+  There are no guarantees that things won't break if you do this.
 
 The default backend is an implementation detail, but is currently `MicroMamba`.
 

src/backend.jl

@@ -7,6 +7,8 @@ function backend()
         end
         if backend == "MicroMamba"
             STATE.backend = :MicroMamba
+        elseif backend == "Null"
+            STATE.backend = :Null
         elseif backend == "System"
             ok = false
             for exe in (exe == "" ? ["micromamba", "mamba", "conda"] : [exe])
@@ -36,6 +38,9 @@ function conda_cmd(args=``; io::IO=stderr)
     b = backend()
     if b == :MicroMamba
         MicroMamba.cmd(args, io=io)
+    elseif b == :Null
+        @error "Backend is 'Null' - no action taken"
+        return nothing
     elseif b == :System
         `$(STATE.condaexe) $args`
     else

src/env.jl

@@ -9,6 +9,7 @@ This file defines functions for interacting with the environment, such as `withe
 "Activate" the Conda environment by modifying the given dict of environment variables.
 """
 function activate!(e)
+    backend() == :Null && return e
     old_path = get(e, "PATH", "")
     d = envdir()
     path_sep = Sys.iswindows() ? ';' : ':'