Allow / in GitVersion (#39398)

This commit allows version specifiers to refer to git branches that contain
forward slashes. For example, the following is valid syntax now:

    pkg@git.releases/1.0
   
It also adds a new method `Spec.format_path(fmt)` which is like `Spec.format`,
but also maps unsafe characters to `_` after interpolation. The difference is
as follows:

    >>> Spec("pkg@git.releases/1.0").format("{name}/{version}")
    'pkg/git.releases/1.0'

    >>> Spec("pkg@git.releases/1.0").format_path("{name}/{version}")
    'pkg/git.releases_1.0'

The `format_path` method is used in all projections. Notice that this method
also maps `=` to `_`

    >>> Spec("pkg@git.main=1.0").format_path("{name}/{version}")
    'pkg/git.main_1.0'
   
which should avoid syntax issues when `Spec.prefix` is literally copied into a
Makefile as sometimes happens in AutotoolsPackage or MakefilePackage

lib/spack/llnl/util/filesystem.py

@@ -156,6 +156,37 @@ def lookup(name):
     shutil.copystat = copystat
 
 
+def polite_path(components: Iterable[str]):
+    """
+    Given a list of strings which are intended to be path components,
+    generate a path, and format each component to avoid generating extra
+    path entries.
+
+    For example all "/", "\", and ":" characters will be replaced with
+    "_". Other characters like "=" will also be replaced.
+    """
+    return os.path.join(*[polite_filename(x) for x in components])
+
+
+@memoized
+def _polite_antipattern():
+    # A regex of all the characters we don't want in a filename
+    return re.compile(r"[^A-Za-z0-9_.-]")
+
+
+def polite_filename(filename: str) -> str:
+    """
+    Replace generally problematic filename characters with underscores.
+
+    This differs from sanitize_filename in that it is more aggressive in
+    changing characters in the name. For example it removes "=" which can
+    confuse path parsing in external tools.
+    """
+    # This character set applies for both Windows and Linux. It does not
+    # account for reserved filenames in Windows.
+    return _polite_antipattern().sub("_", filename)
+
+
 def getuid():
     if sys.platform == "win32":
         import ctypes