internal: simplify structpath to eager initialize

[denik]

Changes

Simplify structpath by removing lazy initialization.

Why

Easier to extend this package.

Tests

Existing tests.

Ran benchmarks on structdiff and structwalk, there are no noticeable changes.

[eng-dev-ecosystem-bot]

Run: 17645885672

	Env	✅​pass	❌​FAIL	🔄​flaky	🙈​skip
✅​	aws linux	311			525
✅​	aws windows	312			524
❌​	aws-ucws linux	417	2	4	423
❌​	aws-ucws windows	420	2	2	422
✅​	azure linux	311			524
🔄​	azure windows	309		3	523
❌​	azure-ucws linux	421	2		422
❌​	azure-ucws windows	422	2		421
✅​	gcp linux	310			526
✅​	gcp windows	311			525

10 failing tests:

Test Name	aws-ucws linux	aws-ucws windows	azure windows	azure-ucws linux	azure-ucws windows
TestAccept	❌​FAIL	❌​FAIL	🔄​flaky	❌​FAIL	❌​FAIL
TestAccept/bundle/deploy/lakebase/synced-database-table	❌​FAIL	❌​FAIL	🙈​skip	❌​FAIL	❌​FAIL
TestAccept/bundle/templates/default-python/integration_classic	✅​pass	✅​pass	🔄​flaky	✅​pass	✅​pass
TestAccept/bundle/templates/default-python/integration_classic/DATABRICKS_CLI_DEPLOYMENT=terraform/UV_PYTHON=3.12	✅​pass	✅​pass	🔄​flaky	✅​pass	✅​pass
TestFilerWorkspaceNotebook	✅​pass	🔄​flaky	✅​pass	✅​pass	✅​pass
TestFilerWorkspaceNotebook/rJupyterNb.ipynb	✅​pass	🔄​flaky	✅​pass	✅​pass	✅​pass
TestFsCpFileToDirFileNotOverwritten	🔄​flaky	✅​pass	✅​pass	✅​pass	✅​pass
TestFsCpFileToDirFileNotOverwritten/local_to_uc-volumes	🔄​flaky	✅​pass	🙈​skip	✅​pass	✅​pass
TestFsCpFileToFileFileNotOverwritten	🔄​flaky	✅​pass	✅​pass	✅​pass	✅​pass
TestFsCpFileToFileFileNotOverwritten/uc-volumes_to_dbfs	🔄​flaky	✅​pass	🙈​skip	✅​pass	✅​pass