refactor(wit): discover worlds and package with the AST parser

[walterjgsp]

Item 3 of #66 (wit side). Depends on the shared parser from #73, already synced here.

DiscoverWorlds/DiscoverPackage scanned lines with worldRegex/packageRegex. The package regex did not match package wasi:cli@0.2.0; at all (returned empty), and a line scan can't tell a comment or a second declaration on the same line. They now use ast.ParseFileLenient (skips use/variant/flags, which discovery doesn't need).

• Behaviour kept: non-recursive directory scan in sorted order, worlds deduplicated in first-seen order, first package wins, a missing path is an error.
• Behaviour changed on purpose: a file that does not parse is an error naming the file (generation parses strictly anyway); before, it was skipped silently.
• tools/please_wit/ast is removed; imports, BUILD deps and docs point to tools/common/wit/ast.
• Removed the tests of the deleted regex helpers; added 5 for what they got wrong (versioned package, comments, two worlds on a line, unmodelled declarations, syntax error).

plz test //tools/please_wit/... //tools/common/wit/...: 325 passed.

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