fix sizegen so it handles type aliases

[systay]

Description

The sizegen code generator wasn't handling type aliases well, so make sizegen was failing:

2024/08/26 09:31:48 unhandled type: *types.Alias
panic: interface conversion: types.Type is *types.Alias, not *types.Named

goroutine 1 [running]:
main.(*sizegen).generateType.func1({0x741a58, 0xc09ef28840})
	/home/runner/work/vitess/vitess/go/tools/sizegen/sizegen.go:152 +0x6f
main.findImplementations(0xc0949f9140, 0xc09e30c1e0, 0xc000031890)
	/home/runner/work/vitess/vitess/go/tools/sizegen/sizegen.go:176 +0xee
main.(*sizegen).generateType(0xc000031ce8, 0xc0949f91a0, 0xc051cbe7b0, 0xc09e320e00)
	/home/runner/work/vitess/vitess/go/tools/sizegen/sizegen.go:150 +0x285
main.(*sizegen).generateKnownType(0xc000031ce8, 0xc09e320e00)

This PR fixes sizegen so it can handle type aliases.

Related Issue(s)

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

[vitess-bot]

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[codecov]

Codecov Report

Attention: Patch coverage is 71.42857% with 2 lines in your changes missing coverage. Please review.

Project coverage is 68.97%. Comparing base (e6843dc) to head (e82c10f).
Report is 1 commits behind head on main.

Files	Patch %	Lines
go/tools/sizegen/sizegen.go	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16650      +/-   ##
==========================================
- Coverage   68.99%   68.97%   -0.03%     
==========================================
  Files        1562     1562              
  Lines      200754   200760       +6     
==========================================
- Hits       138508   138468      -40     
- Misses      62246    62292      +46     

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[vmg]

Nice and clean 😎🙌

[GuptaManan100]

Thank you!