feature: adds modules and visibility

[baszalmstra]

This (still way too large, Im sorry 😓 ) PR adds use of pub keywords and resolved visibility. It also introduces the concept of modules which are ordered in a hierarchical manner based on their location on disk. This is required to be able to specify visibility scopes. This PR also (again Im sorry for the size 😓 ) introduces the concept of definitions from all over the package in the form of PackageDefs. This can later be used to more easily implement use as well.

All of this required quite the overhaul, especially because we have modules now instead of files.

Closes #248

This PR is still in Draft because I want to:

• add diagnostics for missing module files (foo/ exists but no mod.rs or foo.rs). We decided that that is actually not an error.
• add more tests!
• test visibility resolution
• Actually check visibility in paths

[codecov]

Codecov Report

Merging #283 into master will increase coverage by 0.39%.
The diff coverage is 80.42%.

@@            Coverage Diff             @@
##           master     #283      +/-   ##
==========================================
+ Coverage   79.34%   79.73%   +0.39%     
==========================================
  Files         220      228       +8     
  Lines       13069    13500     +431     
==========================================
+ Hits        10369    10764     +395     
- Misses       2700     2736      +36     

Impacted Files	Coverage Δ
crates/mun_compiler_daemon/src/lib.rs	0.00% <0.00%> (ø)
crates/mun_hir/src/item_tree/tests.rs	100.00% <ø> (ø)
crates/mun_language_server/src/diagnostics.rs	0.00% <0.00%> (ø)
crates/mun_language_server/src/main_loop.rs	23.21% <0.00%> (ø)
crates/mun_syntax/src/ast.rs	80.00% <ø> (ø)
crates/mun_syntax/src/ast/generated.rs	76.14% <ø> (+0.91%)	⬆️
...ates/mun_syntax/src/parsing/grammar/expressions.rs	94.02% <ø> (ø)
crates/mun_syntax/src/parsing/grammar/patterns.rs	90.47% <ø> (ø)
crates/mun_syntax/src/parsing/grammar/types.rs	100.00% <ø> (ø)
crates/mun_test/src/fixture.rs	92.85% <ø> (ø)
... and 84 more

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[Wodann]

Very happy to see this work nearing completion. Mun v0.3 is shaping up nicely!

This is only a partial review, but I wanted to make sure that part of my comments were already actionable. I'll continue the review later.

[Wodann]

The next batch of reviewed files 😅

[baszalmstra]

I think I solved all issues, also rebased everything on current master.

[Wodann]

The last batch of comments. Hopefully we can merge this soon 🌲