Cleanup import resolution logic

[cd1m0]

As of #87 we changed solc-typed-ast to infer the set of all imported files, and pass them in to the compiler in the standard JSON. The idea was that we could support a uniform interface between wasm and native compilers - since one of them (wasm) has support for resolving callbacks, but the native one doesn't. Also this way compilers don't need to look around the file system or do anything else - we give them everything they need to compile.

The logic of finding all of the imports however is a little more complex as described here. Our (ok, ok my) original code was a little hacky. In this PR we clean it up to follow more precisely the algorithm described in the docs.

Specifically:

1. The keys in the map files computed in findAllFiles are now proper source unit names as defined in docs.
2. findAllFiles was cleaned up a little ,and now no-longer returns a new map.
3. We pass the original remappings to the underlying compilers again (this is needed so that the underlying compiler computes the same source unit names as we do, and finds them in the files map)

[codecov-commenter]

Codecov Report

Merging #93 (eda0998) into master (6d537a4) will decrease coverage by 0.05%.
The diff coverage is 90.62%.

@@            Coverage Diff             @@
##           master      #93      +/-   ##
==========================================
- Coverage   92.26%   92.20%   -0.06%     
==========================================
  Files         254      254              
  Lines        5195     5197       +2     
  Branches      817      819       +2     
==========================================
- Hits         4793     4792       -1     
- Misses        253      254       +1     
- Partials      149      151       +2     

Impacted Files	Coverage Δ
src/compile/input.ts	78.57% <ø> (ø)
src/compile/inference/imports.ts	90.90% <88.88%> (-5.17%)	⬇️
src/compile/utils.ts	91.30% <100.00%> (-0.15%)	⬇️

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[blitz-1306]

I approve. One small question, that is not a blocker and not affect approach overall.

[blitz-1306]

Looks good