Attempt to speed up initial directory/glob correspondence

Attempt to speed up initial directory/glob correspondence

In https://github.com/joaotavora/eglot/issues/602, not only a new glob processing system was implemented, but
also a new, more correct, way to look for directories that might hold
files matched by one of these globs.

Answering this question is important because the file watchers for
'workspace/didChangeWatchedFiles' are placed on a per-directory basis.
Previously, a glob such as /foo/**/bar/*.el would fail to produce
practical file-watching effects because /foo/**/bar/ isn't really a
directory.

However, answering this question is also expensive, as the globs sent
by the LSP server are meant to match files, not directories.  The only
way is to list all files under the project's root directory and test
each glob on each one.  If it matches at least one file, that file's
directory is meant to be watched.

We suspect that in https://github.com/joaotavora/eglot/issues/645 and https://github.com/joaotavora/eglot/issues/633 we are falling victim to LSP server
who serve a tremendous unoptimized number of globs, one for each file.
So instead of sending just '/foo/**/bar/*.el' they send
'/foo/**/bar/quux.el', '/foo/**/bar/quuz.el', etc...  which would
tremendeously slow down the process.  But this is only a suspicion.

This commit tries some simple optimizations: if a directory is known
to be watch-worthy becasue one of its files matched a single glob, no
more files under that directory are tried.  This should help somewhat.

Also fixed a bug in 'eglot--files-recursively', though I suspect that
doesn't make that much of a difference.

* eglot.el (eglot--directories-matched-by-globs): New helper.
(eglot--files-recursively): Fix bug.

GitHub-reference: per https://github.com/joaotavora/eglot/issues/645