Some fixes to precompile "replay"

[KristofferC]

The previous strategy was to import all loaded packages into the "PrecompileStagingArea" and then execute the precompile statements. Here, I instead try to execute the precompile statement and if this throws an UndefVarError I try import that symbol from the loaded modules. If there exist multiple loaded modules with that name, we ignore the statement.

As a future improvement to ignoring the precompile statement could be to try executeing it with the different modules with the same name, one by one, until we manage to execute something that works.

Fixes #798
Fixes #767

With this PR I can precompile a sysimage of OmniPackage.jl which didn't work before.

[codecov]

Codecov Report

Merging #805 (8ad085d) into master (696064c) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #805   +/-   ##
=======================================
  Coverage   82.98%   82.98%           
=======================================
  Files           3        3           
  Lines         670      670           
=======================================
  Hits          556      556           
  Misses        114      114           

Impacted Files	Coverage Δ
src/PackageCompiler.jl	92.99% <100.00%> (ø)

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