Regression. The prettier tests will fail until expandGlob() is fixed
since they try to match absolute paths outside of CWD.

Should help later when extracting common functionality to expandGlob(),
like exclusion. Makes findTestModules() an async generator.

Adds an exclude field to ExpandGlopOptions. This should be the final
API.

In other words, support path prefixes when specifying excludes. This
aligns with the file matchers of the test runner and prettier. It's less
redundant since this cannot be achieved efficiently on the client-end.

It will help with expanding **

fs/glob.ts:
- Improve prototypes for expandGlob() and expandGlobSync() from denoland#604.
- Rename glob() to globToRegExp().
- Add normalizeGlob() and joinGlobs().
- Extract GlobToRegExpOptions from GlobOptions, remove the strict
  and filepath options.

fs/globrex.ts:
- Add GlobrexOptions.

fs/path/constants.ts:
- Add SEP_PATTERN.

fs/walk.ts:
- Add WalkOptions::includeFiles
- Default WalkOptions::includeDirs to true.
- Don't traverse directories matching a skip pattern.
- Remove walkSync()'s default root value.

prettier:
- Refactor to use expandGlob().

testing:
- Make findTestModules() an async generator.