Fix import-before-module check

[barucden]

PR #51635 introduced function check_src_module_wrap which checks whether a module declaration is missing or whether a using/import is declared before a module.

The original implementation, based on manual checks via regexes, did not catch all possible cases. In particular, it did not consider multi-line comments, e.g.,

#=
using Foo
=#
module Bar
end

threw errors even though it should be valid.

Single-line comments, multi-line comments, and docstrings make it quite difficult to reliably check that a module is declared and not preceded by any using/import. Some examples are

#= =# using A
module B end

#= #=
=#
using A
=#
module B end

"docs" module B end

Because of that, this commit avoids all manually-written, specialized conditions and employs Meta.parse to parse the code into Exprs. It then becomes very simple to check the condition of interest. The downside is increased runtime.

[WITHOUT the shortcut; see bellow] I measured the time it takes to check three files of different sizes (the first measurements is the current version, the second is master):

 # 63979 lines of code
@btime Base.check_src_module_wrap(Base.PkgId("dummy"), "LibSCIP.jl")
  102.267 ms (535826 allocations: 36.19 MiB)
  5.630 μs (8 allocations: 528 bytes)

 # 1051 lines of code
@btime Base.check_src_module_wrap(Base.PkgId("dummy"), "Primes.jl")
  3.525 ms (21503 allocations: 1.08 MiB)
  5.946 μs (9 allocations: 656 bytes)

 # 79 lines of code
@btime Base.check_src_module_wrap(Base.PkgId("dummy"), "BenchmarkTools.jl")
  106.170 μs (578 allocations: 46.02 KiB)
  3.240 μs (8 allocations: 560 bytes)

The slowdown is dramatic (most of the time is spent reading the source file into a String).

I added a condition that checks if the file content starts with "module ", in which case the function returns immediatelly. This shortcut helps: LibSCIP.jl takes only 5.865 μs (instead of 102.265 ms). Of course, the shortcut is taken only in some cases...

I am submitting this PR anyway for discussion (CC: original author @IanButterworth).

Fixes #55792

[IanButterworth]

Thanks for trying to fix this. Maybe there's a way to do this in Base.require, checking that it's not importing packages into Main during precompilation.

[IanButterworth]

See #55824