Add a new command for removing temporary files

[bhwqy]

Problem statement
I find two problems after removing temporary files as faq:

• Tools in downloads/tools needs download again.
• Source code is missing when debugging with external code.

Solution

Add a new command to remove temporary files instead of removing manually.

Alternative solution
For the first problem, it is better to move downloads/tools to installed/tools. Related cmake files I find are

• vcpkg_acquire_msys.cmake
• vcpkg_find_acquire_program.cmake

For the second problem, extract source file to installed/src. Related cmake file I find is

• vcpkg_extract_source_archive_ex.cmake