bug#41646: Startup in Windows is very slow when load-path contains many

[Stefan Monnier via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

> The `load-hints` can reduce searching attempts by putting the matched
> paths on the top of `load-path`, it won't break the original
> load-path; and the patch for the `package.el` will put the installed
> files into the `load-hints`.

The downside is that it can break existing setups for users who use
`package.el` but also modify their `load-path` "by hand" in the
init file, and it doesn't help users who don't use `package.el`.

Note also that your `load-hints` could grow large, so scanning it could
take a significant amount of time.  Maybe it would make sense to turn it
into a hash table for those entries that don't use the "*" special
thingy (and maybe use a radix-tree for those entries using the "*"
special thingy)?

But your prefix idea makes me think maybe we can aim for a significantly
smaller table, where we basically record only one entry per
package/directory, like for "~/.emacs.d/elpa/helm-core-VERSION/" we just
record "helm" because all the `.el` files share the "helm" prefix.
I.e. keep for each dir the corresponding longest-common-prefix.
If we're careful to consider only those files with a `.el` suffix, then
I think we can reduce the hint to such a longest-common-prefix.  I.e. an
info which doesn't say just "you can find FOO* files here" but "you can
find *only* FOO* files here".

Then we should be able to create quickly (so it can be recomputed on the
fly whenever `load-path` changes) a radix-tree that maps a relative
file name to the list of directories from `load-path` where it is
worthwhile to look (by filtering out those dirs whose
longest-common-prefix doesn't match).  We'd only do it for MUST_SUFFIX
is specified, of course.

> 1. On my local linux test env, disable load-hints on the package.el,
> the test cli spends 6.327s; and enable load-hints then it spends
> 5.392s.
>
> 2. On my local Windows test env, disable load-hints on the package.el,
> the test cli spends 11.769s, and enable load-hints then it spends
> 7.279s.

Is that with or without using `package-quickstart`?

BTW, in your patch, you change `locate-file-internal` which seems wrong,
since that function is not specific to loading ELisp files, it's also
used for $MANPATH, $PATH, and things like that.

Similarly, I wasn't able to convince myself that your patch does the
right thing when `require` or `load` is used such that MUST_SUFFIX is
not specified.


        Stefan