Prefix parent package name to extension names

[ven-k]

• This prevents clash of extension names.
• Currently, clashes with ConstructionBase.jl.
• • And clashing names throw fatal errors while building sysimages.
• See: Prefix parent package name to extension names ConstructionBase.jl#78

[aplavin]

#90
Wasn't this fixed in PackageCompiler already at JuliaLang/PackageCompiler.jl#805 and JuliaLang/PackageCompiler.jl#767? I thought those fixes made #90 obsolete.

[ven-k]

I saw there were merge conflicts; so opened this.

Changes from PackageCompiler PR 805 aren't part of release yet¹. That must be why I ran into this issue.
With PackageCompiler#master, I could build the sysimage despite the clash. So once that is released, these changes won't be essential.

However, it is still a better practice. Note that example in Pkg.jl's docs on extension, now prefixes parent package name². So I'm keeping this open, if you want to merge it.

Footnotes

1. https://github.com/JuliaLang/PackageCompiler.jl/compare/v2.1.5...master ↩

2. https://pkgdocs.julialang.org/dev/creating-packages/#Conditional-loading-of-code-in-packages-(Extensions) ↩

[jw3126]

@ven-k if you bump the patch version, I can tag a release with this PR included.