RFR: 8300140: ZipFile.isSignatureRelated returns true for files in META-INF subdirectories [v11]

[Eirik Bjorsnos]

> Some call sites of SignatureFileVerifier.isBlockOrSF fails to check that files reside in META-INF directly, and not in a subdirectory of META-INF.
> 
> The mentioned call sites needs updates to check and ignore such files.
> 
> A new test VerifyUnrelatedSignatureFiles is added which verifies that [*.SF, *.RSA] files in META-INF/ subdirectories are indeed ignored.

Eirik Bjorsnos has updated the pull request incrementally with two additional commits since the last revision:

 - The accidental regression where MANIFEST.MF was incorrecly not matched by SignatureFileVerifier.isSigningRelated revealed was not caught by any test. Adding a sanity check to IgnoreUnrelatedSignatureFiles which verifies that a basic signed JAR has the expected manifest sections.
 - The update of SignatureFileVerifier.isSigningRelated to call isInMetaInf accidentally removed "META-INF/" prefix stripping. This broke some checks in the remainder of this method.
   
   I chose to restore current behaviour by comparing MANIFEST.MF and "SIG-" files using the full path. This saves the allocation caused by substring and also makes sure isBlockOrSF now always called with the full path.

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11976/files
  - new: https://git.openjdk.org/jdk/pull/11976/files/c3a19971..34873625

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11976&range=10
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11976&range=09-10

  Stats: 21 lines in 2 files changed: 17 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/11976.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11976/head:pull/11976

PR: https://git.openjdk.org/jdk/pull/11976