Consuming system requirements only when building a package (#3980)

* Add consuming system packages

Signed-off-by: Uilian Ries <[email protected]>

* Reorganize system package manager structure

Signed-off-by: Uilian Ries <[email protected]>

* Simplify system reqs only when building example

Signed-off-by: Uilian Ries <[email protected]>

---------

Signed-off-by: Uilian Ries <[email protected]>

Author: uilianries

Diff for: examples/tools.rst

@@ -13,4 +13,4 @@ Conan recipe tools examples
    tools/autotools/autotools
    tools/scm/git/capture_scm/git_capture_scm
    tools/microsoft/msbuild
-   tools/system/package_manager
+   tools/system/system

Diff for: examples/tools/system/system.rst

@@ -0,0 +1,13 @@
+.. _examples_system:
+
+
+System Packages
+===============
+
+
+.. toctree::
+   :maxdepth: 2
+
+
+   system_package/package_manager
+   system_package/consuming_system_packages

Diff for: examples/tools/system/system_package/consuming_system_packages.rst

@@ -0,0 +1,65 @@
+.. _examples_tools_system_consuming_system_packages:
+
+Consuming system requirements only when building a package
+==========================================================
+
+In some cases, you may want to consume system requirements only when building a package, but not when installing it.
+It can be useful when you want to build a package in a CI/CD pipeline, but you don't want to run the system package
+manager when installing the Conan package in a different environment.
+For those cases, there are few approaches that can be used to achieve this goal.
+
+
+Consume a Conan package wrapper for a system package as build requirement
+-------------------------------------------------------------------------
+
+In this approach, you can use a Conan package for a :ref:`wrapped system package<examples_tools_system_package_manager>`.
+Then, the package can be consumed regularly by the method
+:ref:`build_requirements()<reference_conanfile_methods_build_requirements>`.
+
+.. code-block:: python
+
+    from conan import ConanFile
+
+    class MyPackage(ConanFile):
+        name = "mypackage"
+        settings = "os", "compiler", "build_type", "arch"
+
+        def build_requirements(self):
+            self.tool_requires("ncurses/system")
+
+        ...
+
+This ensures that downstream consumers of the package *mypackage* will not directly invoke the system
+package manager (e.g., apt-get). Only the direct package consumer of the system wrap package for ``ncurses``
+will execute the system package manager when building the package.
+
+Centralizing and wrapping ``ncurses`` in a separated recipe makes it reusable across multiple cases and is
+good practice to avoid code duplication.
+
+
+Consume the system package directly in the build() method
+---------------------------------------------------------
+
+In case wanting to run the system package manager only when building the package, but not having a Conan package to
+wrap the system library information, it's possible to run the system package manager in the **build()** method:
+
+.. code-block:: python
+
+    from conan import ConanFile
+    from conan.tools.system import package_manager
+
+    class MyPackage(ConanFile):
+        settings = "os", "compiler", "build_type", "arch"
+        ...
+
+        def build(self):
+            if self.settings.os == "Linux":
+                apt = package_manager.Apt(self)
+                apt.install(["libncurses-dev"], update=True, check=True)
+
+This way, the system package manager will be called only when building the package, not when installing it.
+There is the advantage of not needed to create a separated Conan package to wrap the system library information,
+this is a much simpler case, when only a single recipe need to install the system package.
+
+Still, this approach may lead to code duplication if multiple recipes consume the same system package.
+It is recommended to use this method sparingly and only for well-contained cases.