Disallow partial regex matches when searching for JAR file to extract

[Ruin0x11]

For example, if a library class is named Validator, there could be another class named ValidatorContext found by the JAR file searching logic first, causing Emacs to jump to the wrong file. The server will open the correct file, though. This may also cause the error End of buffer to be displayed if the offset in the correct file is greater than the file size of the incorrect file.