fix: give alternate unit suffixes their own keys

[Sanjays2402]

Fixes #1250

When a UNECE symbol column lists aliases (% or pct), the generator emitted both descriptors under the same Python key, so the second overwrote the first — units.PERCENT resolved to pct and the % descriptor was unreachable as a module attribute. KILOGRAM_PER_LITRE, DECITONNE and RACK_UNIT had the same collision.

units_from_xls.py now keeps the canonical key for the first suffix and derives a distinct key for each alternate (PERCENT_PCT, etc.); units.py is regenerated to match. Lookup via units.Unit('%') / units.Unit('pct') is unchanged. New test/util/units_test.py fails on master and passes with the fix.

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[Sanjays2402]

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[Sanjays2402]

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[Sanjays2402]

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This should be ready for review whenever a maintainer has a moment. Happy to rebase or adjust anything if needed. Thanks!

[glados-verma]

There are a few problems with this approach, including:

1. The alias names have "jitter" e.g. KILOGRAM_PER_LITRE_KG_PER_L
2. The approach doesn't care for which one should be considered the "primary" alias

Given the small number of units that have aliases, we might as well create a lookup such units.

[Sanjays2402]

Both fair, and the jitter one was the bigger problem — KILOGRAM_PER_LITRE_KG_PER_L is not a name anyone would type.

Reworked in b81fdd5 along the lines you suggested. The generator now keeps the canonical key for the first symbol listed in the sheet (that's the primary one) and appends alternates to ALL_UNITS anonymously, so they stay reachable via units.Unit('kg/L') / Unit('dtn') / Unit('RU') but get no module-level name. A small explicit ALIAS_UNIT_KEYS table names the one alias I think is genuinely worth exposing, PERCENT_PCT, since pct is common in test configs — it's a two-line dict so adding another later is trivial.

Net effect on the generated module: PERCENT is % again, KILOGRAM_PER_LITRE is kg/l, DECITONNE is dt, RACK_UNIT is U, and none of the four are overwritten by their alias. Diff to units.py is now 4 hunks instead of the previous key-renaming spread.

Tests updated to match (primary keeps its symbol / alternates resolve by lookup only). 4 pass here; they fail 3/4 against current master.

Happy to drop PERCENT_PCT too and make the rule purely "only the primary symbol gets a name" if you'd rather have zero exceptions.