added Atlas helper tool (#18497)

* added Atlas helper tool
* further improvements

tools/atlas/atlas.md

@@ -0,0 +1,77 @@
+# Atlas Package Cloner
+
+Atlas is a simple package cloner tool that automates some of the
+workflows and needs for Nim's stdlib evolution.
+
+Atlas is compatible with Nimble in the sense that it supports the Nimble
+file format.
+
+
+## How it works
+
+Atlas uses git commits internally; version requirements are translated
+to git commits via `git show-ref --tags`.
+
+Atlas uses URLs internally; Nimble package names are translated to URLs
+via Nimble's  `packages.json` file.
+
+Atlas does not call the Nim compiler for a build, instead it creates/patches
+a `nim.cfg` file for the compiler. For example:
+
+```
+############# begin Atlas config section ##########
+--noNimblePath
+--path:"../nimx"
+--path:"../sdl2/src"
+--path:"../opengl/src"
+############# end Atlas config section   ##########
+```
+
+The version selection is deterministic, it picks up the *minimum* required
+version. Thanks to this design, lock files are not required.
+
+
+## Dependencies
+
+Dependencies are neither installed globally, nor locally into the current
+project. Instead a "workspace" is used. The workspace is the nearest parent
+directory of the current directory that does not contain a `.git` subdirectory.
+Dependencies are managed as **siblings**, not as children. Dependencies are
+kept as git repositories.
+
+Thanks to this setup, it's easy to develop multiple projects at the same time.
+
+A project plus its dependencies are stored in a workspace:
+
+  $workspace / main project
+  $workspace / dependency A
+  $workspace / dependency B
+
+
+No attempts are being made at keeping directory hygiene inside the
+workspace, you're supposed to create appropriate `$workspace` directories
+at your own leisure.
+
+
+## Commands
+
+Atlas supports the following commands:
+
+
+### Clone <url>
+
+Clones a URL and all of its dependencies (recursively) into the workspace.
+Creates or patches a `nim.cfg` file with the required `--path` entries.
+
+
+### Clone <package name>
+
+The `<package name>` is translated into an URL via `packages.json` and
+then `clone <url>` is performed.
+
+
+### Search <term term2 term3 ...>
+
+Search the package index `packages.json` for a package that the given terms
+in its description (or name or list of tags).
+