Fix multiple issues with Python in views (#42601)

This fixes bugs, performance issues, and removes no longer necessary code.

Short version:

1. Creating views from Python extensions would error if the Spack `opt` dir itself was in some symlinked directory. Use of `realpath` would expand those, and keying into `merge_map` would fail.
2. Creating views from Python extensions (and Python itself, potentially) could fail if the `bin/` dir contains symlinks pointing outside the package prefix -- Spack keyed into `merge_map[target_of_symlink]` incorrectly.
3. In the `python` package the `remove_files_from_view` function was broken after a breaking API change two years ago (#24355). However, the entire function body was redundant anyways, so solved it by removing it.
4. Notions of "global view" (i.e. python extensions being linked into Python's own prefix instead of into a view) are completely outdated, and removed. It used to be supported but was removed years ago.
5. Views for Python extension would _always_ copy non-symlinks in `./bin/*`, which is a big mistake, since all we care about is rewriting shebangs of scripts; we don't want to copy binaries. Now we first check if the file is executable, and then read two bytes to check if it has a shebang, and only if so, copy the entire file and patch up shebangs.

The bug fixes for (1) and (2) basically consist of getting rid of `realpath` entirely, and instead simply keep track of file identifiers of files that are copied/modified in the view. Only after patching up regular files do we iterate over symlinks and check if they target one of those. If so, retarget it to the modified file in the view.

lib/spack/llnl/util/filesystem.py

@@ -920,29 +920,35 @@ def get_filetype(path_name):
     return output.strip()
 
 
-@system_path_filter
-def is_nonsymlink_exe_with_shebang(path):
-    """
-    Returns whether the path is an executable script with a shebang.
-    Return False when the path is a *symlink* to an executable script.
-    """
+def has_shebang(path):
+    """Returns whether a path has a shebang line. Returns False if the file cannot be opened."""
     try:
-        st = os.lstat(path)
-        # Should not be a symlink
-        if stat.S_ISLNK(st.st_mode):
-            return False
-
-        # Should be executable
-        if not st.st_mode & (stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH):
-            return False
-
-        # Should start with a shebang
         with open(path, "rb") as f:
             return f.read(2) == b"#!"
-    except (IOError, OSError):
+    except OSError:
         return False
 
 
+@system_path_filter
+def is_nonsymlink_exe_with_shebang(path):
+    """Returns whether the path is an executable regular file with a shebang. Returns False too
+    when the path is a symlink to a script, and also when the file cannot be opened."""
+    try:
+        st = os.lstat(path)
+    except OSError:
+        return False
+
+    # Should not be a symlink
+    if stat.S_ISLNK(st.st_mode):
+        return False
+
+    # Should be executable
+    if not st.st_mode & (stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH):
+        return False
+
+    return has_shebang(path)
+
+
 @system_path_filter(arg_slice=slice(1))
 def chgrp_if_not_world_writable(path, group):
     """chgrp path to group if path is not world writable"""