Refactor dependency management

[kryksyh]

This PR moves all third-party dependencies into a separate submodule: muse_deps.

External dependencies are now declared in the manifest file: ExtDepsManifest.cmake.

• I signed the CLA as username:
• The title of the PR describes the problem it addresses.
• Each commit's message describes its purpose and effects, and references the issue it resolves. If changes are extensive, there is a sequence of easily reviewable commits.
• The code in the PR follows the coding rules.
• I understand all aspects of the code I'm contributing and I'm able to explain it if requested.
• The code compiles and runs on my machine, preferably after each commit individually. I have manually tested and verified that my changes fulfil their intended purpose.
• No prior attempts to resolve this problem exist, or if they do, I listed them in my PR description and described how I avoided repeating past mistakes.
• There are no unnecessary changes. [i hope so]
• I created a unit test or vtest to verify the changes I made (if applicable).

Build configuration

audacity: audacity/audacity/master
audacity platforms: linux_x64
musescore: musescore/MuseScore/main
musescore platforms: linux_x64