Avoid duplicate stat calls during startup/loading (#55331)

Avoids immediately successive stat calls for the same path during
startup & loading.

According to MacOS Instruments this reduces `stat64` calls during
`--start=no -e "using Pkg"` from 844 to 672.

On MacOS I don't see a speed improvement, but on WSL2 @timholy reported
the test from #55171 sees a
modest improvement.

This PR (10 iterations)
```
tim@diva:~/.julia/bin$ time for i in {1..10}; do ./cli --help &> /dev/null; done

real    0m2.999s
user    0m3.547s
sys     0m6.552s
```

master
```
real    0m3.217s
user    0m3.794s
sys     0m6.700s
```
1.11-rc2:
```
real    0m3.746s
user    0m4.169s
sys     0m6.755s
```

I left the `@debug`s in as they might be useful for similar
investigations.

base/client.jl

@@ -339,11 +339,13 @@ function _global_julia_startup_file()
     # If it is not found, then continue on to the relative path based on Sys.BINDIR
     BINDIR = Sys.BINDIR
     SYSCONFDIR = Base.SYSCONFDIR
+    p1 = nothing
     if !isempty(SYSCONFDIR)
         p1 = abspath(BINDIR, SYSCONFDIR, "julia", "startup.jl")
         isfile(p1) && return p1
     end
     p2 = abspath(BINDIR, "..", "etc", "julia", "startup.jl")
+    p1 == p2 && return nothing # don't check the same path twice
     isfile(p2) && return p2
     return nothing
 end

base/loading.jl

@@ -822,14 +822,15 @@ end
 # given a project directory (implicit env from LOAD_PATH) and a name,
 # find an entry point for `name`, and see if it has an associated project file
 function entry_point_and_project_file(dir::String, name::String)::Union{Tuple{Nothing,Nothing},Tuple{String,Nothing},Tuple{String,String}}
-    path = normpath(joinpath(dir, "$name.jl"))
-    isfile_casesensitive(path) && return path, nothing
     dir_name = joinpath(dir, name)
     path, project_file = entry_point_and_project_file_inside(dir_name, name)
     path === nothing || return path, project_file
     dir_jl = dir_name * ".jl"
     path, project_file = entry_point_and_project_file_inside(dir_jl, name)
     path === nothing || return path, project_file
+    # check for less likely case with a bare file and no src directory last to minimize stat calls
+    path = normpath(joinpath(dir, "$name.jl"))
+    isfile_casesensitive(path) && return path, nothing
     return nothing, nothing
 end
 
@@ -3809,7 +3810,7 @@ end
                 end
                 if !ispath(f)
                     _f = fixup_stdlib_path(f)
-                    if isfile(_f) && startswith(_f, Sys.STDLIB)
+                    if _f != f && isfile(_f) && startswith(_f, Sys.STDLIB)
                         continue
                     end
                     @debug "Rejecting stale cache file $cachefile because file $f does not exist"
@@ -3831,13 +3832,14 @@ end
                         return true
                     end
                 else
-                    fsize = filesize(f)
+                    fstat = stat(f)
+                    fsize = filesize(fstat)
                     if fsize != fsize_req
                         @debug "Rejecting stale cache file $cachefile because file size of $f has changed (file size $fsize, before $fsize_req)"
                         record_reason(reasons, "include_dependency fsize change")
                         return true
                     end
-                    hash = isdir(f) ? _crc32c(join(readdir(f))) : open(_crc32c, f, "r")
+                    hash = isdir(fstat) ? _crc32c(join(readdir(f))) : open(_crc32c, f, "r")
                     if hash != hash_req
                         @debug "Rejecting stale cache file $cachefile because hash of $f has changed (hash $hash, before $hash_req)"
                         record_reason(reasons, "include_dependency fhash change")

base/stat.jl

@@ -184,8 +184,14 @@ macro stat_call(sym, arg1type, arg)
 end
 
 stat(fd::OS_HANDLE)         = @stat_call jl_fstat OS_HANDLE fd
-stat(path::AbstractString)  = @stat_call jl_stat  Cstring path
-lstat(path::AbstractString) = @stat_call jl_lstat Cstring path
+function stat(path::AbstractString)
+    # @info "stat($(repr(path)))" exception=(ErrorException("Fake error for backtrace printing"),stacktrace())
+    @stat_call jl_stat  Cstring path
+end
+function lstat(path::AbstractString)
+    # @info "lstat($(repr(path)))" exception=(ErrorException("Fake error for backtrace printing"),stacktrace())
+    @stat_call jl_lstat Cstring path
+end
 if RawFD !== OS_HANDLE
     global stat(fd::RawFD)  = stat(Libc._get_osfhandle(fd))
 end