Added `public_dependencies` including test.

[GPMueller]

Closes #101.

TODO:

• [x] dependencies are now private by default
• [x] public_dependencies are visible to dependent targets
• [x] implement tests of this feature
• [x] clear docs of the updated inheritance rules for includes and flags

Note: we may want to find a more elegant solution at some point, but this one should work fine for now.

---

The rules are as follows:

public dependencies of a shared lib

app -> A -> X:

• app gets flags and include dirs: public of A and X
• app links to: A

app -> A -> Y:

• app gets flags and include dirs: public of A and Y
• app links to: A

public dependencies of a static lib

app -> B -> M:

• app gets flags and include dirs: public of B and M, interface of B
• app links to: B and M

app -> B -> N:

• app gets flags and iinclude dirs: public and interface of B and N
• app links to: B and N

private dependencies of a shared lib

app -> A -> X:

• app gets flags and iinclude dirs: public of A
• app links to: A

app->A->Y:

• app gets flags and iinclude dirs: public of A
• app links to: A

private dependencies of a static lib

app -> B -> M:

• app gets flags and iinclude dirs: public and interface of B and M
• app links to: B, M

app -> B -> N:

• app gets flags and iinclude dirs: public and interface of B and N
• app links to: B, N

[codecov[bot]]

Codecov Report

Merging #127 (f61b9f3) into master (9c3dda0) will decrease coverage by 0.47%. The diff coverage is 97.05%.

@@            Coverage Diff             @@
##           master     #127      +/-   ##
==========================================
- Coverage   78.42%   77.95%   -0.48%     
==========================================
  Files          16       18       +2     
  Lines        1154     1279     +125     
==========================================
+ Hits          905      997      +92     
- Misses        249      282      +33     

Impacted Files	Coverage Δ
clang_build/target.py	80.32% <94.11%> (+1.39%)	:arrow_up:
clang_build/directories.py	100.00% <100.00%> (ø)
clang_build/project.py	83.67% <100.00%> (+0.45%)	:arrow_up:
clang_build/cli.py	51.61% <0.00%> (ø)
clang_build/__init__.py	100.00% <0.00%> (ø)
clang_build/environment.py	92.10% <0.00%> (+3.21%)	:arrow_up:
clang_build/toolchain.py	82.29% <0.00%> (+5.20%)	:arrow_up:
clang_build/single_source.py	63.26% <0.00%> (+8.16%)	:arrow_up:

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