gh-37650: src/sage/features/sagemath.py: Add feature SAGE_SRC
    
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This feature is for conditionalizing some doctests.

Fixes doctest failure reported in
#37645.

### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation accordingly.

### ⌛ Dependencies

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URL: #37650
Reported by: Matthias Köppe
Reviewer(s): François Bissey, Gonzalo Tornaría, Matthias Köppe

Author: Release Manager

src/sage/features/sagemath.py

@@ -43,6 +43,31 @@
 from .join_feature import JoinFeature
 
 
+class SAGE_SRC(StaticFile):
+    r"""
+    A :class:`~sage.features.Feature` which describes the presence of the
+    monolithic source tree of the Sage library.
+
+    """
+    def __init__(self):
+        r"""
+        TESTS::
+
+            sage: from sage.features.sagemath import SAGE_SRC
+            sage: isinstance(SAGE_SRC(), SAGE_SRC)
+            True
+        """
+        from sage.env import SAGE_SRC
+        # We check the file bin/sage-src-env-config.in, which by design is:
+        # - never installed,
+        # - not included in the sagemath-standard sdist,
+        # - included only in one modularized sdist, of pkgs/sage-conf_pypi,
+        #   where it appears in a subdirectory (sage_root/src/bin/)
+        StaticFile.__init__(self, 'SAGE_SRC',
+                            filename='bin/sage-src-env-config.in',
+                            search_path=(SAGE_SRC,) if SAGE_SRC else ())
+
+
 class sagemath_doc_html(StaticFile):
     r"""
     A :class:`~sage.features.Feature` which describes the presence of the documentation
@@ -1095,7 +1120,8 @@ def all_features():
         sage: list(all_features())
         [...Feature('sage.combinat'), ...]
     """
-    return [sagemath_doc_html(),
+    return [SAGE_SRC(),
+            sagemath_doc_html(),
             sage__combinat(),
             sage__geometry__polyhedron(),
             sage__graphs(),

src/sage/misc/package_dir.py

@@ -111,6 +111,7 @@ def read_distribution(src_file):
 
     EXAMPLES::
 
+        sage: # needs SAGE_SRC
         sage: from sage.env import SAGE_SRC
         sage: from sage.misc.package_dir import read_distribution
         sage: read_distribution(os.path.join(SAGE_SRC, 'sage', 'graphs', 'graph_decompositions', 'tdlib.pyx'))

src/sage/misc/replace_dot_all.py

@@ -111,8 +111,9 @@ def find_replacements(location, package_regex=None, verbose=False):
 
     EXAMPLES::
 
+        sage: # needs SAGE_SRC
         sage: from sage.misc.replace_dot_all import *
-        sage: location = os.path.join(sage.env.SAGE_SRC, 'sage/plot/arc.py')
+        sage: location = os.path.join(sage.env.SAGE_SRC, 'sage', 'plot', 'arc.py')
         sage: find_replacements(location, package_regex='sage[.]plot[.]all', verbose=True)
         [[..., ..., 'from sage.plot.graphics import Graphics']]
     """
@@ -295,8 +296,9 @@ def process_line(location, line, replacements, row_index, verbose=False):
 
     Replacing the first line which needs a replacement in the source file with filepath ``src/sage/plot/arc.py``::
 
+        sage: # needs SAGE_SRC
         sage: from sage.misc.replace_dot_all import *
-        sage: location = os.path.join(sage.env.SAGE_SRC, 'sage/plot/arc.py')
+        sage: location = os.path.join(sage.env.SAGE_SRC, 'sage', 'plot', 'arc.py')
         sage: replacements = find_replacements(location, package_regex='sage[.]plot[.]all', verbose=True); replacements
         [[477, 24, 'from sage.plot.graphics import Graphics']]
         sage: with open(location, "r") as file:
@@ -401,6 +403,7 @@ def walkdir_replace_dot_all(dir, file_regex=r'.*[.](py|pyx|pxi)$', package_regex
 
     EXAMPLES::
 
+        sage: # needs SAGE_SRC
         sage: from sage.misc.replace_dot_all import *
         sage: walkdir_replace_dot_all(os.path.join(sage.env.SAGE_SRC, 'sage'))  # not tested
     """

src/sage_setup/clean.py

@@ -80,6 +80,7 @@ def _find_stale_files(site_packages, python_packages, python_modules, ext_module
     course. We check that when the doctest is being run, that is,
     after installation, there are no stale files::
 
+        sage: # needs SAGE_SRC
         sage: from sage.env import SAGE_SRC, SAGE_LIB, SAGE_ROOT
         sage: from sage_setup.find import _cythonized_dir
         sage: cythonized_dir = _cythonized_dir(SAGE_SRC)
@@ -98,6 +99,7 @@ def _find_stale_files(site_packages, python_packages, python_modules, ext_module
 
     TODO: Also check extension modules::
 
+        sage: # needs SAGE_SRC
         sage: stale_iter = _find_stale_files(SAGE_LIB, python_packages, python_modules, [], extra_files)
         sage: from importlib.machinery import EXTENSION_SUFFIXES
         sage: skip_extensions = tuple(EXTENSION_SUFFIXES)

src/sage_setup/find.py

@@ -1,3 +1,4 @@
+# sage.doctest: needs SAGE_SRC
 """
 Recursive Directory Contents
 """