Extend `dfetch import` to detect CMake FetchContent & ExternalProject

[ben-edna]

Description

dfetch import currently generates a manifest by scanning Git submodules and SVN externals only.
Many modern CMake‑based projects manage dependencies via: [dfetch.rea...thedocs.io]

• FetchContent_Declare()
• FetchContent_MakeAvailable()
• ExternalProject_Add()

It would be extremely helpful if Dfetch could detect these CMake dependency declarations and convert them into dfetch.yaml entries.

Requested Feature

Add optional CMake parsing to dfetch import, for example:

dfetch import --detect-cmake

This should:

1. Parse CMakeLists.txt and included .cmake files.
2. Extract URL, GIT_REPOSITORY, and GIT_TAG information (preferablly all options [externalproject_add](of https://cmake.org/cmake/help/latest/module/ExternalProject.html#command:externalproject_add).
3. Generate equivalent projects: entries in the manifest.
4. Avoid modifying CMake files unless a future flag (--patch-cmake) is introduced.

Example

From:

FetchContent_Declare(json
    GIT_REPOSITORY https://github.com/nlohmann/json.git
    GIT_TAG v3.11.2
)

To:

projects:
  - name: json
    url: https://github.com/nlohmann/json.git
    tag: v3.11.2

Why

This allows seamless migration from CMake‑managed dependencies to Dfetch’s unified, VCS‑agnostic vendoring approach.