wip: add a `create_distribution` entry point

[KristofferC]

This creates basically a new julia "distribution" with a custom set of packages in it.

Right now:

• packages get added to the sysimage
• packages are treated as stdlibs so they can be loaded via using Package without a corresponding project file
• packages are also treated as stdlibs by Pkg
• package's source code is not copied, shims are added so that Pkg will properly treat it as an stdlib

TODO:

• [ ] test
• [ ] see how extensions are treated

[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 64 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 79.86%. Comparing base (c1e37c4) to head (4cc409b).

Files with missing lines	Patch %	Lines
src/PackageCompiler.jl	0.00%	64 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1073      +/-   ##
==========================================
- Coverage   86.21%   79.86%   -6.35%     
==========================================
  Files           3        3              
  Lines         805      869      +64     
==========================================
  Hits          694      694              
- Misses        111      175      +64     

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