RFR: 8332226: "Invalid package name:" from source launcher

[Christian Stein]

Please review this change excluding directory paths with invalid elements when computing the packages of a module in source-launch mode.

Note that this inital iteration of the change only fixes the in-memory computation of package names - the behaviour of the associated module reader implementation is untouched for the time being. This introduces a difference in the set of resources being readable at runtime. For example, see the modified test case: the new and invalid `.bar` directory doesn't show up in the contents of the module - but it is reported by the module reader's `list()` method.

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Commit messages:
 - 8332226: "Invalid package name:" from source launcher

Changes: https://git.openjdk.org/jdk/pull/19245/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19245&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332226
  Stats: 23 lines in 2 files changed: 19 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19245.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19245/head:pull/19245

PR: https://git.openjdk.org/jdk/pull/19245