maint: add a dependency check to coverage

This commit implements a dependency check to the `maint/coverage`
script, that checks for the rustup, python3, and grcov binaries in
$PATH; the bs4 and lxml python packages; and the llvm-tools Rust
component.

Fixes #776

maint/coverage

@@ -30,6 +30,29 @@ cd "$TOPDIR"
 UNIT=no
 INTEGRATION=no
 
+# Check if all dependencies are satisfied
+DEPS_BINARY=("rustup" "python3" "grcov")
+DEPS_PYTHON=("bs4" "lxml")
+
+for bin in "${DEPS_BINARY[@]}"; do
+    if ! type "$bin" > /dev/null;  then
+        echo "Missing $bin in PATH"
+        exit 1
+    fi
+done
+
+for pkg in "${DEPS_PYTHON[@]}"; do
+    if ! python3 -c "import $pkg" > /dev/null; then
+        echo "Missing python package $pkg"
+        exit 1
+    fi
+done
+
+if ! rustup component list --installed | grep llvm-tools &> /dev/null; then
+    echo "Missing llvm-tools in the Rust toolchain"
+    exit 1
+fi
+
 while getopts "ch" opt ; do
     case "$opt" in
 	h) usage