llnl.util.filesystem: multiple entrypoints and max_depth (#47495)

If a package `foo` doesn't implement `libs`, the default was to search recursively for `libfoo` whenever asking for `spec[foo].libs` (this also happens automatically if a package includes `foo` as a link dependency).

This can lead to some strange behavior:
1. A package that is normally used as a build dependency (e.g. `cmake` at one point) is referenced like
   `depends_on(cmake)` which leads to a fully-recursive search for `libcmake` (this can take
   "forever" when CMake is registered as an external with a prefix like `/usr`, particularly on NFS mounts).
2. A similar hang can occur if a package is registered as an external with an incorrect prefix

- [x] Update the default library search to stop after a maximum depth (by default, search
  the root prefix and each directory in it, but no lower).
- [x] 

The following is a list of known changes to `find` compared to `develop`:

1. Matching directories are no longer returned -- `find` consistently only finds non-dirs, 
   even at `max_depth`
2. Symlinked directories are followed (needed to support max_depth)
3. `find(..., "dir/*.txt")` is allowed, for finding files inside certain dirs. These "complex"
   patterns are delegated to `glob`, like they are on `develop`.
4. `root` and `files` arguments both support generic sequences, and `root`
   allows both `str` and `path` types. This allows us to specify multiple entry points to `find`.

---------

Co-authored-by: Peter Scheibel <scheibel1@llnl.gov>

1 files changed, 2 insertions, 2 deletions

diff --git a/var/spack/repos/builtin.mock/packages/attributes-foo/package.py b/var/spack/repos/builtin.mock/packages/attributes-foo/package.py
index 31c88f4b08..b882fc9b65 100644
--- a/var/spack/repos/builtin.mock/packages/attributes-foo/package.py
+++ b/var/spack/repos/builtin.mock/packages/attributes-foo/package.py
@@ -44,7 +44,7 @@ class AttributesFoo(BundlePackage):
     # Header provided by the bar virutal package
     @property
     def bar_headers(self):
-        return find_headers("bar/bar", root=self.home.include, recursive=False)
+        return find_headers("bar", root=self.home.include, recursive=True)
 
     # Libary provided by the bar virtual package
     @property
@@ -59,7 +59,7 @@ class AttributesFoo(BundlePackage):
     # Header provided by the baz virtual package
     @property
     def baz_headers(self):
-        return find_headers("baz/baz", root=self.baz_home.include, recursive=False)
+        return find_headers("baz", root=self.baz_home.include, recursive=True)
 
     # Library provided by the baz virtual package
     @property