big Pkg rename: Pkg, Pkg2 => Pkg1, Pkg; sysimg loads only Pkg(2).

Most of the functionality of Pkg1 has now be replicated in Pkg2 and
seems to be far more stable and usable than Pkg1 at this point, so
this renames them and makes the system image only include the new
Pkg(2). You can still use Pkg1 by explicitly loading it:

    include(joinpath(JULIA_HOME,"../../base/pkg1.jl"))

Hopefully all of Pkg1's functionality will be subsumed shortly.
Pkg(2): some cosmetic fixups and style changes.
Pkg(2): rename runbuildscript => build. Yay, brevity.
Pkg(2): don't checkout commits we don't have when releasing a package.

If we don't have commits for any registered commits, hit the network
to get them. This should hopefully address @timholy's issue in #3695.
remove test.c. closes #4071
fix #4075, memory bug in generated code
minor additions to NEWS.md
add test for #4075 and fix signed/unsigned comparison
omit numeric type decorations such as "f0" in print, showcompact, and print_shortest
show() Float16 as "float16(2.5)"
closes #4061
update modules doc to include default "importall Base.Operators"
other minor NEWS and doc updates
make Reverse immutable

I would kind of prefer to delete this since it doesn't really work.
some tiny flisp updates

pad symbol allocations to 8 bytes. malloc is probably doing this internally
anyway, but this makes valgrind happier.
found a gc_handle that looks like it should be made earlier. this probably
changes nothing, but couldn't hurt.
Fix #4077
Fix beta function for BigFloats
accurate sind, cosd functions
Merge pull request #4078 from JuliaLang/anj/beta

Fix beta function for BigFloats
Merge branch 'master' of github.com:JuliaLang/julia
`using Nonexistent`: better behavior loading unfound names.

    julia> using FooBar
    ERROR: FooBar not found
     in require at loading.jl:45

The core change is that `find_in_path(name)` now returns `nothing`
when `name` isn't found and various functions that call it must
check explicitly for `nothing` as a return value and throw an error
as appropriate. In particular `require` and `reload` now raise an
error instead of just forging ahead trying to load a file we know
doesn't exist, which resulted in less-than-clear error messages.
fix #4080, don't try to deepcopy Tasks
Revert "Fix beta function for BigFloats"

This reverts commit 5b3745294c8cfd4bb3c33ff2482ae256f5e138b7.