gh-35716: sage.functions: Decouple from sage.symbolic.expression …

…and `mpmath`, update `# needs`

    
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### 📚 Description
`sage.functions` advertises itself as providing symbolic functions, but
it also provides the mechanism that dispatches `sin(1.0)` to field
methods such as `RealNumber.sin`, `RealBall.sin` etc.

Here we make the modules from `sage.functions` at least *importable*
when the symbolic subsystem (pynac) is not available. (Evaluating the
functions still goes through the symbolic subsystem as of this PR.) This
is achieved by using `lazy_import` and by moving the symbol table from
`sage.symbolic.expression` (containing pynac) to a new pure Python
module `sage.symbolic.symbols`.

We also use `lazy_import` for imports from `mpmath` and mutual imports
from other modules in `sage.functions`.

A `lazy_import` of `RR` added here reveals a weakness of
`sage.misc.dev_tools.find_objects_from_name` regarding `lazy_import`s
(because `RR` happens to be used in an example for this function.) We
fix it here.

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- Part of: #29705
- Cherry-picked from: #35095
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- [x] 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: #35716
Reported by: Matthias Köppe
Reviewer(s): Kwankyu Lee, Matthias Köppe

src/sage/calculus/calculus.py

@@ -429,7 +429,7 @@
 from sage.misc.parser import Parser, LookupNameMaker
 from sage.structure.element import Expression
 from sage.symbolic.ring import var, SR
-from sage.symbolic.expression import symbol_table
+from sage.symbolic.symbols import symbol_table
 from sage.symbolic.function import Function
 from sage.symbolic.function_factory import function_factory
 from sage.symbolic.integration.integral import (indefinite_integral,