[release-1.11] Fix KeyError in require_stdlib for non-stdlib extensions

[IanButterworth]

When require_stdlib is called with an extension and the 'from' module is not from the bundled depot (from_stdlib=false), start_loading was not called but end_loading was still called in the finally block, causing a KeyError when trying to pop from package_locks.

Track whether start_loading was called and only call end_loading when appropriate.

Fixes #60392

Reproducer (test added):

% julia +1.11.8 --startup-file=no -e '
    Pkg_pkgid = Base.PkgId(Base.UUID("44cfe95a-1eb2-52ea-b672-e2afdf69b78f"), "Pkg")
    REPLExt_pkgid = Base.PkgId(Base.uuid5(Pkg_pkgid.uuid, "REPLExt"), "REPLExt")
    FakeREPL = Module(:REPL)
    FakeREPL_pkgid = Base.PkgId(Base.UUID("3fa0cd96-eef1-5676-8a61-b3b8758bbffb"), "REPL")
    Base.register_root_module(FakeREPL)
    Base.require_stdlib(Pkg_pkgid, "REPLExt", FakeREPL)
    Base.maybe_root_module(REPLExt_pkgid) isa Module || error("")
    println("Success!")
'
ERROR: KeyError: key Base.PkgId(Base.UUID("e5eb5ef1-03cf-53a7-ae1d-5a66b08e832b"), "REPLExt") not found
Stacktrace:
 [1] pop!
   @ ./dict.jl:583 [inlined]
 [2] end_loading
   @ ./loading.jl:2140 [inlined]
 [3] macro expansion
   @ ./loading.jl:2724 [inlined]
 [4] macro expansion
   @ ./lock.jl:273 [inlined]
 [5] require_stdlib(package_uuidkey::Base.PkgId, ext::String, from::Module)
   @ Base ./loading.jl:2689
 [6] top-level scope
   @ none:7

[IanButterworth]

@DilumAluthge @KristofferC I believe this is relatively urgent?

[DilumAluthge]

I've recreated the backports-release-1.11 branch.

[DilumAluthge]

@vtjnash Would you be able to review this?

[DilumAluthge]

I tested this PR locally, and at least for me it seems to fix the issue reported in #60392.

On Julia 1.11.8 (with using Revise in startup.jl):

$ julia +1.11
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.11.8 (2025-11-06)
 _/ |\__'_|_|_|\__'_|  |  Official https://julialang.org/ release
|__/                   |

(@v1.11) pkg> Unhandled Task ERROR: KeyError: key Base.PkgId(Base.UUID("e5eb5ef1-03cf-53a7-ae1d-5a66b08e832b"), "REPLExt") not found
Stacktrace:
 [1] pop!
   @ ./dict.jl:583 [inlined]
 [2] end_loading
   @ ./loading.jl:2140 [inlined]
 [3] macro expansion
   @ ./loading.jl:2724 [inlined]
 [4] macro expansion
   @ ./lock.jl:273 [inlined]
 [5] require_stdlib(package_uuidkey::Base.PkgId, ext::String, from::Module)
   @ Base ./loading.jl:2689
 [6] load_pkg()
   @ REPL ~/juliaup.depot/juliaup/julia-1.11.8+0.aarch64.apple.darwin14/share/julia/stdlib/v1.11/REPL/src/Pkg_beforeload.jl:8
 [7] (::REPL.var"#123#141"{REPL.LineEdit.MIState, REPL.LineEditREPL, REPL.LineEdit.Prompt})()
   @ REPL ~/juliaup.depot/juliaup/julia-1.11.8+0.aarch64.apple.darwin14/share/julia/stdlib/v1.11/REPL/src/REPL.jl:1284

Using this PR:

$ julia +pr60491
               _
   _       _ _(_)_     |  Documentation: https://docs.julialang.org
  (_)     | (_) (_)    |
   _ _   _| |_  __ _   |  Type "?" for help, "]?" for Pkg help.
  | | | | | | |/ _` |  |
  | | |_| | | | (_| |  |  Version 1.11.8 (2025-12-26)
 _/ |\__'_|_|_|\__'_|  |  IanButterworth:ib/1.11_fix_require_stdlib/37e76389047 (fork: 730 commits, 688 days)
|__/                   |

(@v1.11) pkg> st
Status `~/.julia/environments/v1.11/Project.toml`
  [295af30f] Revise v3.13.0

(@v1.11) pkg>

[vtjnash]

SGTM