Refactor locs, dependencies into generic sub build

[LeStarch]

Originating Project/Creator
Affected Component
Affected Architectures(s)
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Has Unit Tests (y/n)
Builds Without Errors (y/n)
Unit Tests Pass (y/n)
Documentation Included (y/n)

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Change Description

Creates generic sub-builds and makes locs/dependencies into a sub-build. This replaces the hardcoded prescan function.

Rationale

Sub-builds are a standard pattern in CMake and this exposes this functionality in a generic form to users. Reworking pre-scan into this has the following advantages:

1. "Rebuilding" loc/dependencies does not take substantial time after the first run
2. Proper dependency tracking and rebuilds work
3. -j will work for sub/builds and fpp-depend isn't mapped to the whole CPU.

Future Work

Note any additional work that will be done relating to this issue.

Review fixes tracked as check-list:

• Remove commented out FPrime-Code
• Remove comment link to pre-scan
• Remove Fprime-Code.cmake
• Change [prescan] to [subbuild] or [subbuild-name]
• Always print executing step

[LeStarch]

Note: although structurally much nicer, this slows down the initial generate time. Here are some numbers:

Original (Ref): initial generation
-- Configuring done (14.0s)
-- Generating done (0.6s)

This PR (Ref): initial generation w/ 16 jobs
-- Configuring done (17.7s)
-- Generating done (0.6s)

Original (Ref): re-generation
-- Configuring done (12.7s)
-- Generating done (0.5s)

This PR (Ref): initial re-generation w/ 16 jobs
-- Configuring done (6.0s)
-- Generating done (0.6s)

Notice that the re-generation (changed a model/CMakeLists.txt) is substantially faster.

[thomas-bc]

A couple of nits/comments, but overall looks good to me!

[thomas-bc]

🚀