Add `import_to_main` flag for Main module package name conflicts resolution

[krynju]

This contains only the import_to_main part of https://github.com/JuliaLang/PackageCompiler.jl/pull/794

Fixes https://github.com/JuliaLang/PackageCompiler.jl/issues/768

Changelog:

1. added import_into_main flag (default true), which allows you to control whether the top level packages will be imported into Main in the sysimage
2. added skip & warnings on the import to Main name conflict https://github.com/JuliaLang/PackageCompiler.jl/issues/768

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 84.64%. Comparing base (8e57908) to head (c68f67b).

:exclamation: Current head c68f67b differs from pull request most recent head c255bdc. Consider uploading reports for the commit c255bdc to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #807   +/-   ##
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  Coverage   84.64%   84.64%           
=======================================
  Files           3        3           
  Lines         827      827           
=======================================
  Hits          700      700           
  Misses        127      127           

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[krynju]

@KristofferC Can we merge and tag this? I confirmed it works fine on my case