ax_code_coverage.m4: simplify CODE_COVERAGE_OUTPUT_*

This is going to be used by the follow-up commit.

* m4/ax_code_coverage.m4 (CODE_COVERAGE_OUTPUT_FILE): Change
to coverage.info.
(CODE_COVERAGE_OUTPUT_DIRECTORY): Change to coverage.

m4/ax_code_coverage.m4

@@ -173,11 +173,9 @@ code-coverage-clean:
 #    Multiple directories may be specified, separated by whitespace.
 #    (Default: $(top_builddir))
 #  - CODE_COVERAGE_OUTPUT_FILE: Filename and path for the .info file generated
-#    by lcov for code coverage. (Default:
-#    $(PACKAGE_NAME)-$(PACKAGE_VERSION)-coverage.info)
+#    by lcov for code coverage. (Default: coverage.info)
 #  - CODE_COVERAGE_OUTPUT_DIRECTORY: Directory for generated code coverage
-#    reports to be created. (Default:
-#    $(PACKAGE_NAME)-$(PACKAGE_VERSION)-coverage)
+#    reports to be created. (Default: coverage)
 #  - CODE_COVERAGE_BRANCH_COVERAGE: Set to 1 to enforce branch coverage,
 #    set to 0 to disable it and leave empty to stay with the default.
 #    (Default: empty)
@@ -206,8 +204,8 @@ code-coverage-clean:
 
 # Optional variables
 CODE_COVERAGE_DIRECTORY ?= $(top_builddir)
-CODE_COVERAGE_OUTPUT_FILE ?= $(PACKAGE_NAME)-$(PACKAGE_VERSION)-coverage.info
-CODE_COVERAGE_OUTPUT_DIRECTORY ?= $(PACKAGE_NAME)-$(PACKAGE_VERSION)-coverage
+CODE_COVERAGE_OUTPUT_FILE ?= coverage.info
+CODE_COVERAGE_OUTPUT_DIRECTORY ?= coverage
 CODE_COVERAGE_BRANCH_COVERAGE ?=
 CODE_COVERAGE_LCOV_SHOPTS_DEFAULT ?= $(if $(CODE_COVERAGE_BRANCH_COVERAGE),\
 --rc lcov_branch_coverage=$(CODE_COVERAGE_BRANCH_COVERAGE))