Precompile/import into main flags & warnings for package name conflicts

[krynju]

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 disable_precompile flag (default false), which allows you to turn off the precompile part of the build completely
3. added skip & warnings on the precompile name conflict Precompile stage doesn't support packages with the same names and different uuids #767
4. added skip & warnings on the import to Main name conflict Incremental sysimage build doesn't support packages with same name but different uuid #768

While this doesn't fix the #767 and #768 issues it makes them not error, which in most cases will be fine.
If not then the warning should point the user at some possible solution.
The current solutions to both of these problems are either using the flags added in this PR or modifying the precompile statements.

[codecov]

Codecov Report

Merging #794 (978106f) into master (9b6b461) will decrease coverage by 0.85%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #794      +/-   ##
==========================================
- Coverage   83.08%   82.23%   -0.85%     
==========================================
  Files           3        3              
  Lines         668      670       +2     
==========================================
- Hits          555      551       -4     
- Misses        113      119       +6     

Impacted Files	Coverage Δ
src/PackageCompiler.jl	92.61% <100.00%> (-0.55%)	⬇️

... and 1 file with indirect coverage changes

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[staticfloat]

FWIW, using import_into_main = false is sufficient for me to build this example case.